Multicylinder internal combustion engine, inter-cylinder air/fuel ratio imbalance determination apparatus, and method therefor

ABSTRACT

A multicylinder engine includes: combustion chambers; fuel injection valves corresponding to the individual combustion chambers; an exhaust control catalyst; an upstream-side air/fuel ratio sensor disposed upstream of the exhaust control catalyst; and a downstream-side A/F sensor disposed downstream of the catalyst. A first abnormality determination-purpose rich A/F control of controlling the A/F of the mixture formed in each combustion chamber to an A/F richer than stoichiometric is executed when it needs to be determined whether the downstream-side A/F sensor is abnormal. An inter-cylinder A/F imbalance determination of estimating the A/F of the mixture in each combustion chamber based on the output of the upstream-side A/F sensor is executed when the first abnormality determination-purpose rich A/F control is being executed, and it is determined whether there is a difference between the estimated A/Fs of the mixtures.

The disclosure of Japanese Patent Application No. 2010-27674 filed on Feb. 10, 2010, including the specification, drawings and abstract is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a multicylinder internal combustion engine, and to a determination apparatus and a determination method that determine the presence or absence of inter-cylinder air/fuel ratio imbalance.

2. Description of the Related Art

In a multicylinder internal combustion engine in which fuel injection valves are disposed corresponding to each one of a plurality of combustion chambers so that fuel is injected from the fuel injection valves into the corresponding combustion chambers, it is preferable that the amounts of fuel injected from the fuel injection valves be all controlled to equal and optimum amounts, in order to minimize the exhaust emissions discharged from the combustion chambers. That is, in the multicylinder internal combustion engine, for example, in the case where the amount of fuel to be injected from the fuel injection valves is set at an amount that will minimize the exhaust emissions discharged from the combustion chambers (i.e., an optimum amount), there is a possibility of the exhaust emissions from the combustion chambers increasing unexpectedly if the amount of fuel injected from any one of the fuel injection valves is not controlled to the foregoing optimum amount.

Therefore, in the foregoing multicylinder internal combustion engine, if there occurs an event in which the amount of fuel injected from any one of the fuel injection valves is not controlled to the optimum amount, that is, in which there is a difference among the air/fuel ratios of the mixtures formed in the combustion chambers, that is, a difference in the air/fuel ratio of the mixture among the combustion chambers, it is very important to know about that event, from the viewpoint of minimizing the exhaust emissions discharged from the combustion chambers.

An apparatus for detecting the event in which there is a difference in the air/fuel ratio of the mixture among the combustion chambers (hereinafter, referred to as “inter-cylinder air/fuel ratio imbalance”) is disclosed in U.S. Pat. No. 7,152,594.

In the apparatus disclosed in U.S. Pat. No. 7,152,594, an air/fuel ratio sensor that detects the air/fuel ratio of exhaust gas by detecting the oxygen concentration in the exhaust gas is disposed in the exhaust passageway. This apparatus compares the frequency of changes in the output value of the air/fuel ratio sensor with a predetermined reference value, and determines that an inter-cylinder air/fuel ratio imbalance is present if the frequency of changes in the output value of the air/fuel ratio sensor has deviated from a predetermined reference value concerned therewith. Alternatively, the apparatus compares the length of the signal trace of the output of the air/fuel ratio sensor with a predetermined reference value, and determines that an inter-cylinder air/fuel ratio imbalance is present if the length of the signal trace of the output of the air/fuel ratio sensor has deviated from a predetermined reference value concerned therewith.

By the way, in the multicylinder internal combustion engine disclosed in U.S. Pat. No. 7,152,594, the output characteristic of the air/fuel ratio sensor varies depending on the air/fuel ratio of the mixture formed in each combustion chamber (hereinafter, simply referred to as “air/fuel ratio of the mixture”). That is, the output characteristic of the air/fuel ratio sensor varies among the case in which the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio or substantially to the stoichiometric air/fuel ratio, the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is leaner than the stoichiometric air/fuel ratio, and the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio. Therefore, in each of these cases, the accuracy of the determination regarding the inter-cylinder air/fuel ratio imbalance varies.

SUMMARY OF THE INVENTION

The invention provides an apparatus and a method that accurately determines the inter-cylinder air/fuel ratio imbalance in an multicylinder internal combustion engine.

A first aspect of the invention relates to a multicylinder internal combustion engine which includes: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers; and a downstream-side air/fuel ratio sensor disposed in the exhaust passageway downstream of the exhaust control catalyst so as to detect the air/fuel ratio of the exhaust gas that flows out from the exhaust control catalyst; and a control device that executes a first abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the downstream-side air/fuel ratio sensor is abnormal. The multicylinder internal combustion engine further includes a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the first abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.

In the case where the air/fuel ratio of the mixture formed in each of the combustion chambers is estimated on the basis of the output value of the upstream-side air/fuel ratio sensor disposed in the exhaust passageway in order to detect the air/fuel ratio of exhaust gas discharged from the combustion chambers, the accuracy of the estimation is higher when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio than when the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio or to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Therefore, in the case where it is determined whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers through the use of the air/fuel ratios of the mixtures formed in the combustion chambers which are estimated on the basis of output values of the upstream-side air/fuel ratio sensor, the accuracy of the determination is also higher when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio than when the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio or being controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

According to the first aspect of the invention, when the air/fuel ratio of the mixture formed in each combustion chamber is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio in order to determine whether the downstream-side air/fuel ratio sensor has abnormality, the air/fuel ratio of the mixture formed in each combustion chamber is estimated on the basis of the output value of the upstream-side air/fuel ratio sensor. On the basis of the estimated air/fuel ratios of the mixtures, it is determined whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers. Therefore, the accuracy of the determination is high.

Furthermore, according to the first aspect of the invention, it is determined whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers when the air/fuel ratios of the mixtures formed in the combustion chambers are being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio in order to determine whether the downstream-side air/fuel ratio sensor has abnormality. Therefore, an increased accuracy of the determination as to whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers is achieved, and decline of the fuel economy of the internal combustion engine can be restrained.

In the multicylinder internal combustion engine of the first aspect, the control device may execute a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal, and the determination device may execute the inter-cylinder air/fuel ratio imbalance determination when the second abnormality determination-purpose rich air/fuel ratio control is being executed.

With this construction, since the inter-cylinder air/fuel ratio imbalance determination is executed also when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio in order to determine whether the upstream-side air/fuel ratio sensor is abnormal, the frequency of occasions to execute the inter-cylinder air/fuel ratio imbalance determination increases.

Besides, in the foregoing multicylinder internal combustion engine, the determination device may execute the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.

With this construction, the inter-cylinder air/fuel ratio imbalance determination is executed on the basis of output values of the upstream-side air/fuel ratio sensor that is normal, so that the accuracy of the inter-cylinder air/fuel ratio imbalance determination is high.

A second aspect of the invention relates to a multicylinder internal combustion engine comprising: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers; a control device that executes a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal; and a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the second abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.

In the case where the air/fuel ratio of the mixture formed in each of the combustion chambers is estimated on the basis of the output value of the upstream-side air/fuel ratio sensor disposed in the exhaust passageway in order to detect the air/fuel ratio of exhaust gas discharged from the combustion chambers, the accuracy of the estimation is higher when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio than when the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio or to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Therefore, in the case where it is determined whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers through the use of the air/fuel ratios of the mixtures formed in the combustion chambers which are estimated on the basis of output values of the upstream-side air/fuel ratio sensor, the accuracy of the determination is also higher when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio than when the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio or to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

According to the second aspect of the invention, when the air/fuel ratio of the mixture formed in each combustion chamber is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio in order to determine whether the upstream-side air/fuel ratio sensor has abnormality, the air/fuel ratio of the mixture formed in each combustion chamber is estimated on the basis of the output value of the upstream-side air/fuel ratio sensor. On the basis of the estimated air/fuel ratios of the mixtures, it is determined whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers. Therefore, the accuracy of the determination is high.

Furthermore, according to the second aspect of the invention, it is determined whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers when the air/fuel ratios of the mixtures formed in the combustion chambers are being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio in order to determine whether the upstream-side air/fuel ratio sensor has abnormality. Therefore, according to the invention, it is possible to increase the accuracy of the determination as to whether there is a difference in the air/fuel ratio of the mixture among the combustion chambers while restraining decline in the fuel economy of the internal combustion engine.

In the multicylinder internal combustion engine of the second aspect, the determination device may execute the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.

With this construction, the inter-cylinder air/fuel ratio imbalance determination is executed on the basis of output values of the upstream-side air/fuel ratio sensor that is normal, so that the accuracy of the inter-cylinder air/fuel ratio imbalance determination is high.

In the foregoing multicylinder internal combustion engine, the control device may execute at least one of: (1) an engine start-time rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when operation of the multicylinder internal combustion engine is started; (2) a post-fuel injection stop rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when injection of fuel from the fuel injection valves is re-started after the injection of the fuel from the fuel injection valves is stopped; and (3) an exhaust control catalyst-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when temperature of the exhaust control catalyst is higher than a predetermined permissible upper-limit temperature, and the determination device may execute the inter-cylinder air/fuel ratio imbalance determination when the engine start-time rich air/fuel ratio control, the post-fuel injection stop rich air/fuel ratio control or the exhaust control catalyst-purpose rich air/fuel ratio control is being executed.

With this construction, since the inter-cylinder air/fuel ratio imbalance determination is executed when operation of the multicylinder internal combustion engine is started, or when the injection of fuel from the fuel injection valves is re-started after the injection of fuel from the fuel injection valves is stopped, or when the temperature of the exhaust control catalyst is higher than the predetermined permissible upper-limit temperature, the frequency of occasions to execute the inter-cylinder air/fuel ratio imbalance determination increases.

A third aspect of the invention relates to a multicylinder internal combustion engine that includes: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; and an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers. This multicylinder internal combustion engine further includes: a control device that controls the air/fuel ratio of a mixture formed in each of the combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers.

A fourth aspect of the invention relates to an inter-cylinder air/fuel ratio imbalance determination apparatus. This inter-cylinder air/fuel ratio imbalance determination apparatus includes an upstream-side air/fuel ratio sensor disposed in an exhaust passageway upstream of an exhaust control catalyst so as to detect air/fuel ratio of exhaust gas discharged from a plurality of combustion chambers of a multicylinder internal combustion engine. The exhaust control catalyst is disposed in the exhaust passageway so as to remove a specific component of the exhaust gas discharged from the combustion chambers. The inter-cylinder air/fuel ratio imbalance determination apparatus further includes: a control device that controls the air/fuel ratio of a mixture in each of the plurality of combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers.

A fifth aspect of the invention relates to an inter-cylinder air/fuel ratio imbalance determination method that includes: determining whether a first condition for determining whether there is a difference in air/fuel ratio between a plurality of combustion chambers of a multicylinder internal combustion engine is satisfied; determining whether a second condition for controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio between the combustion chambers; detecting the air/fuel ratios of exhaust gas discharged from the plurality of combustion chambers of the multicylinder internal combustion engine; and controlling the air/fuel ratio of the mixture in each of the combustion chambers to the air/fuel ratio richer than the stoichiometric air/fuel ratio when the first condition and the second condition are satisfied, and then determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on the detected air/fuel ratios.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, advantages, and technical and industrial significance of this invention will be described below with reference to the accompanying drawings, in which like numerals denote like elements, and wherein:

FIG. 1 is an overall diagram of a spark ignition type multicylinder internal combustion engine to which an inter-cylinder air/fuel ratio imbalance determination apparatus in accordance with an embodiment of the invention is applied;

FIG. 2 is a diagram showing the exhaust gas purification performances of an upstream-side catalyst and a downstream-side catalyst;

FIG. 3A is a diagram showing an output characteristic of an upstream-side air/fuel ratio sensor, and FIG. 3B is a diagram showing an output characteristic of a downstream-side air/fuel ratio sensor;

FIG. 4 is a diagram showing a map for use for determining the target air/fuel ratio;

FIG. 5 is a diagram showing an example of a flowchart for calculating a duration of injection of fuel from the fuel injection valve;

FIG. 6 is a diagram showing an example of a flowchart for calculating an air/fuel ratio correction coefficient;

FIG. 7 is a diagram showing an example of a flowchart for calculating a stepwise increase value and a stepwise reduction value;

FIG. 8A is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when all the fuel injection valves are normal while the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio, and FIG. 8B is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when the fuel injection valve that corresponds to the first cylinder #1 has an abnormality of injecting an amount of fuel that is larger than a command value of fuel injection amount and the other fuel injection valves are normal while the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio, and FIG. 8C is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when the fuel injection valve that corresponds to the first cylinder #1 has an abnormality of injecting an amount of fuel that is less than the command value of fuel injection amount and the other fuel injection valves are normal while the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio;

FIG. 9A is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when all the fuel injection valve are normal while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio, and FIG. 9B is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when the fuel injection valve that corresponds to the first cylinder #1 has an abnormality of injecting an amount of fuel that is larger than a command value of fuel injection amount and the other fuel injection valves are normal while the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, and FIG. 9C is a diagram showing transition of the output value of the upstream-side air/fuel ratio sensor when the fuel injection valve that corresponds to the first cylinder #1 has an abnormality of injecting an amount of fuel that is less than the command value of fuel injection amount and the other fuel injection valves are normal while the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio;

FIG. 10 is a schematic partial perspective view showing a portion of the upstream-side air/fuel ratio sensor;

FIG. 11 is a partial sectional view showing a portion of the upstream-side air/fuel ratio sensor;

FIG. 12A is a sectional view showing a construction of an air/fuel ratio detection element of the upstream-side air/fuel ratio sensor, and FIG. 12B is a sectional view showing a state of the air/fuel ratio detection element when exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio comes to the air/fuel ratio detection element of the upstream-side air/fuel ratio sensor, and FIG. 12C is a state of the air/fuel ratio detection element when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio comes to the air/fuel ratio detection element of the upstream-side air/fuel ratio sensor;

FIG. 13 is a diagram showing a relation between the air/fuel ratio of exhaust gas that comes to the upstream-side air/fuel ratio sensor and the limiting current value that the upstream-side air/fuel ratio sensor outputs;

FIG. 14 is a diagram showing a flowchart of an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a first embodiment;

FIG. 15 is a diagram showing a flowchart of an example of a routine of executing the setting of an inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the first embodiment;

FIG. 16 is a diagram showing a flowchart showing an example of a routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a second embodiment;

FIG. 17 is a diagram showing a flowchart of an example of a routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a third embodiment;

FIG. 18 is a diagram showing a flowchart of an example of a routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a fourth embodiment;

FIG. 19 is a diagram showing a flowchart of an example of a routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a fifth embodiment;

FIG. 20 is a diagram showing a flowchart of an example of a routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag that shows whether to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with a sixth embodiment;

FIG. 21 is a sectional view showing a construction of an air/fuel ratio detection element that is provided in an upstream-side air/fuel ratio sensor and that has a catalyst; and

FIG. 22 is a diagram showing a construction of a hybrid system.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 is an overall diagram of a spark ignition type multicylinder internal combustion engine (hereinafter, simply referred to as “internal combustion engine”) to which an inter-cylinder air/fuel ratio imbalance determination apparatus in accordance with an embodiment of the invention is applied. The spark ignition type multicylinder internal combustion engine described below is a so-called four-stroke internal combustion engine that cyclically undergoes four strokes, that is, the intake stroke, the compression stroke, the expansion stroke and the exhaust stroke.

The internal combustion engine 10 has a body (hereinafter, referred to as “engine body”) 20. The engine body 20 has a cylinder block and a cylinder head. Besides, the engine body 20 has four combustion chambers 21 each of which is defined by an internal wall surface of a cylinder bore formed in the cylinder block, a top surface of a piston disposed in the cylinder bore, and a lower wall surface of the cylinder head.

FIG. 1 shows a combustion chamber (hereinafter, also referred to as “first cylinder #1”) 21 that is shown at the lowest position, and a combustion chamber (hereinafter, also referred to as “second cylinder #2”) 21 that is shown immediately above the first cylinder #1, and a combustion chamber (hereinafter, also referred to as “third cylinder #3”) 21 that is shown immediately above the second cylinder #2, and a combustion chamber (hereinafter, also referred to as “fourth cylinder #4”) 21 that is shown immediately above the third cylinder #3.

Besides, the cylinder head is provided with intake ports 22 that communicate with the combustion chambers 21. Through the intake ports 22, air is taken into the combustion chambers 21. The intake ports 22 are opened and closed by intake valves (not shown). The cylinder head is also provided with exhaust ports 23 that communicate with the combustion chambers 21. Exhaust gas is discharged from the combustion chambers 21 into the exhaust ports 23. The exhaust ports 23 are opened and closed by exhaust valves (not shown).

Besides, in the cylinder head, ignition plugs 24 are disposed corresponding to the individual combustion chambers 21. Each ignition plug 24 is disposed in the cylinder head so as to be exposed in a corresponding one of the combustion chambers 21 and therefore be able to ignite a mixture of fuel and air, which is formed in the corresponding combustion chamber 21. Furthermore, in the cylinder head, fuel injection valves 25 are disposed corresponding to the intake ports 22. The fuel injection valves 25 are disposed in the cylinder head so as to be exposed in the intake ports 22 and therefore be able to inject fuel into the intake ports 22.

An intake manifold 31 is connected to the intake ports 22. The intake manifold 31 has branch portions each of which is connected to a corresponding one of the intake ports 22, and a surge tank portion where the branch portions meet. Besides, an intake pipe 32 is connected to the surge tank portion of the intake manifold 31. In this embodiment, the intake ports 22, the intake manifold 31 and the intake pipe 32 form an intake passageway 30. An air filter 33 is disposed in the intake pipe 32. A throttle valve 34 is pivotably disposed in the intake pipe 32 between the air filter 33 and the intake manifold 31. An actuator 34 a that drives the throttle valve 34 is connected to the throttle valve 34. The throttle 34 is pivoted by the actuator 34 a so as to change the flow path area in the intake pipe 31, whereby the amount of air taken into the combustion chambers 21 is controlled.

On the other hand, the exhaust ports 23 are connected to an exhaust manifold 41. The exhaust manifold 41 has branch portions 41 a each of which is connected to a corresponding one of the exhaust ports 23, and an exhaust confluence portion 41 b where the branch portions 41 a meet. An exhaust pipe 42 is connected to the exhaust confluence portion 41 b of the exhaust manifold 41. In this embodiment, the exhaust ports 23, the exhaust manifold 41 and the exhaust pipe 42 form an exhaust passageway 40. Besides, an exhaust gas control catalyst 43 that substantially removes specific components from the exhaust gas (hereinafter, referred to as “upstream-side catalyst 43”) is disposed in the exhaust pipe 42. The exhaust pipe 42 downstream of the upstream-side catalyst 43 is provided with an exhaust gas control catalyst 44 that also substantially removes specific components from exhaust gas (hereinafter, referred to as “downstream-side catalyst 44”).

The upstream-side catalyst 43 is a so-called three-way catalyst that is able to simultaneously remove nitrogen oxides (hereinafter, termed NOx), carbon monoxide (hereinafter, termed CO) and hydrocarbons (hereinafter, termed HCs) from exhaust gas at high removal rates when the temperature of the catalyst is higher than a certain temperature (i.e., a so-called activation temperature) and the air/fuel ratio of exhaust gas that flows into the catalyst is within a range X in the vicinity of the stoichiometric air/fuel ratio, as shown in FIG. 2. On the other hand, the upstream-side catalyst 43 has a capability of storing oxygen from exhaust gas when the air/fuel ratio of the exhaust gas that flows into the catalyst is leaner than the stoichiometric air/fuel ratio, and of releasing oxygen stored in the catalyst when the air/fuel ratio of the exhaust gas that flows into the catalyst is richer than the stoichiometric air/fuel ratio (hereinafter, this capability will be referred to as “oxygen storage/release capability”). Therefore, as long as this oxygen storage/release capability functions normally, an internal atmosphere in the upstream-catalyst 43 is maintained substantially in the vicinity of the stoichiometric air/fuel ratio regardless of whether the air/fuel ratio of the exhaust gas that flows into the upstream-side catalyst 43 is leaner or richer than the stoichiometric air/fuel ratio. Therefore, the upstream-side catalyst 43 simultaneously removes NOx, CO and HCs from exhaust gas at high removal rates.

The downstream-side catalyst 44 is also a so-called three-way catalyst, and is able to simultaneously remove NOx, CO and HCs at high removal rates and also has the oxygen storage/release capability, similarly to the upstream-side catalyst 43.

The intake pipe 32 is also provided with an air flow meter 51 that detects the amount of air that flows in the intake pipe 32, that is, the amount of air that is taken into the combustion chambers 21 (hereinafter, this amount of air will be referred to as “intake gas amount”).

Besides, a crank position sensor 53 that detects the rotation phase of a crankshaft (not shown) is disposed on the engine body 20. The crank position sensor 53 outputs a short pulse every time the crank shaft rotates 10°, and outputs a long pulse every time the crankshaft rotates 360°. The rotation speed of the crankshaft, that is, the engine rotation speed, is calculated on the basis these pulses. Besides, an accelerator operation amount sensor 57 detects the amount of depression of an accelerator pedal AP.

The exhaust pipe 42 (the exhaust confluence portion 41 b) upstream of the upstream-side catalyst 43 is provided with an air/fuel ratio sensor (hereinafter, referred to as “upstream-side air/fuel ratio sensor”) 55 that detects the air/fuel ratio of exhaust gas. Furthermore, the exhaust pipe 42 downstream of the upstream-side catalyst 43 but upstream of the downstream-side catalyst 44 is provided with an air/fuel ratio sensor (hereinafter, referred to as “downstream-side air/fuel ratio sensor”) 56 that detects the air/fuel ratio of exhaust gas similarly to the upstream-side air/fuel ratio sensor 43.

The upstream-side air/fuel ratio sensor 55 is a so-called limiting-current type oxygen concentration sensor that outputs an output value I that is smaller the richer the detected air/fuel ratio of exhaust gas and that is greater the leaner the detected air/fuel ratio, as shown in FIG. 3A.

On the other hand, the downstream-side air/fuel ratio sensor 56 is a so-called electromotive force type oxygen concentration sensor that, as show in FIG. 3B, outputs a relatively large constant output value Vg when the detected air/fuel ratio of exhaust gas is richer than the stoichiometric air/fuel ratio, and that outputs a relatively small constant output value Vs when the detected air/fuel ratio of exhaust gas is leaner than the stoichiometric air/fuel ratio. Furthermore, the downstream-side air/fuel ratio sensor 56 outputs an intermediate output value Vm between the relatively large constant output value Vg and the relatively small constant output value Vs when the detected air/fuel ratio of exhaust gas is substantially equal to the stoichiometric air/fuel ratio.

An electric control unit (ECU) 60 shown in FIG. 1 includes a microcomputer having a CPU (microprocessor) 61, a ROM (read-only memory) 62, a RAM (random access memory) 63, a backup RAM 64, an interface 65 that includes an AD converter. These components are interconnected by a bidirectional bus. The interface 65 is connected to the ignition plugs 24, the fuel injection valves 25, and the actuator 34 a of the throttle valve 34. Besides, the air flow meter 51, the crank position sensor 53, the upstream-side air/fuel ratio sensor 55, the downstream-side air/fuel ratio 56, and the accelerator operation amount sensor 57 are also connected to the interface 65.

By the way, in this embodiment, the air/fuel ratio TA/F that is set as a target of the air/fuel ratio of a mixture formed in each of the combustion chambers 21 according to the state of operation of the internal combustion engine, particularly the engine rotation speed and the engine load (hereinafter, the mixture formed in the combustion chambers will be referred to simply as “mixture”), which will hereinafter be referred to simply as “target air/fuel ratio”, is stored beforehand in the electronic control unit 60 in the form of a map of a function between the engine rotation speed N and the engine load L, as shown in FIG. 4. Then, during operation of the internal combustion engine (hereinafter, referred to as “during operation of the engine”), the target air/fuel ratio TA/F commensurate with the engine rotation speed N and the engine load L is retrieved, and the amount of fuel injected from each fuel injection valve 25 (hereinafter, referred to as “fuel injection amount”) is controlled according to the intake gas amount detected by the air flow meter 51 so that the air/fuel ratio of the mixture becomes equal to the target air/fuel ratio. Incidentally, the intake gas amount is controlled by controlling the degree of opening of the throttle valve 34 according to the output demanded of the internal combustion engine.

The control of the fuel injection amount performed when the target air/fuel ratio is the stoichiometric air/fuel ratio and the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio will be described below.

When it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of exhaust gas is leaner than the stoichiometric air/fuel ratio, it means that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio. At this time, according to this embodiment, the fuel injection amount is gradually increased so that the air/fuel ratio of the mixture approaches the stoichiometric air/fuel ratio. On the other hand, when it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of exhaust gas is richer than the stoichiometric air/fuel ratio, it means that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio. At this time, according to this embodiment, the fuel injection amount is gradually decreased so that the air/fuel ratio of the mixture approaches the stoichiometric air/fuel ratio. By controlling the fuel injection amount in this manner, the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio as a whole.

By the way, when the fuel injection amount is controlled as described above, the air/fuel ratio of the mixture changes about the stoichiometric air/fuel ratio, that is, becomes richer than the stoichiometric air/fuel ratio at some times, and becomes leaner than the stoichiometric air/fuel ratio at some other times. In other words, the air/fuel ratio of the mixture oscillates about the stoichiometric air/fuel ratio. From the viewpoint of controlling the air/fuel ratio of the mixture to the stoichiometric air/fuel ratio, it is desirable that the amplitude of the oscillation of the air/fuel ratio of the mixture about the stoichiometric air/fuel ratio be small. Specifically, when the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio, it is desirable to change the air/fuel ratio of the mixture closer to the stoichiometric air/fuel ratio as quickly as possible, and when the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio, it is desirable to change the air/fuel ratio of the mixture closer to the stoichiometric air/fuel ratio as quickly as possible.

Therefore, in this embodiment, when it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture has switched from an air/fuel ratio that is leaner than the stoichiometric air/fuel ratio to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the fuel injection amount is reduced by a relatively large amount in a stepwise manner. According to this operation, when the air/fuel ratio of the mixture has switched from an air/fuel ratio that is leaner than the stoichiometric air/fuel ratio to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the air/fuel ratio of the mixture is changed closer to the stoichiometric air/fuel ratio by a relatively large amount of change. On the other hand, when it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture has switched from an air/fuel ratio that is richer than the stoichiometric air/fuel ratio to an air/fuel ratio that is leaner than the stoichiometric air/fuel ratio, the fuel injection amount is increased by a relatively large amount in a stepwise manner. According to this operation, when the air/fuel ratio of the mixture has switched from an air/fuel ratio that is richer than the stoichiometric air/fuel ratio to an air/fuel ratio that is leaner than the stoichiometric air/fuel ratio, the air/fuel ratio of the mixture is changed closer to the stoichiometric air/fuel ratio by a relatively large amount of change. Thus, the amplitude of oscillation of the air/fuel ratio of the mixture about the stoichiometric air/fuel ratio becomes smaller.

By the way, in order to change the air/fuel ratio of the mixture closer to the stoichiometric air/fuel ratio more quickly, it is desirable that the amount by which the fuel injection amount is reduced in the stepwise manner (hereinafter, the amount will be referred to as “stepwise reduction value”) when the air/fuel ratio of the mixture has switched from the lean side of the stoichiometric air/fuel ratio to the rich side thereof be made larger the larger the difference between the air/fuel ratio of the mixture and the stoichiometric air/fuel ratio which is found when air/fuel ratio of the mixture has switched from the lean side of the stoichiometric air/fuel ratio to the rich side thereof, and it is desirable that the amount by which the fuel injection amount is increased in the stepwise manner (hereinafter, the amount will be referred to as “stepwise increase value”) when the air/fuel ratio of the mixture has switched from the rich side of the stoichiometric air/fuel ratio to the lean side thereof be made larger the larger the difference between the air/fuel ratio of the mixture and the stoichiometric air/fuel ratio which is found when the air/fuel ratio of the mixture has switched from the rich side to the lean side of the stoichiometric air/fuel ratio.

Therefore, in this embodiment, the stepwise reduction value and the stepwise increase value are controlled as follows.

That is, it can be said that the longer the period during which the air/fuel ratio detected by the downstream-side air/fuel ratio sensor 56 remains on the lean side of the stoichiometric air/fuel ratio (hereinafter, referred to as “lean period”), the greater the extent by which the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio. Specifically, the air/fuel ratio of the exhaust gas that flows out of the upstream-side catalyst 43 is considered to be equal to the stoichiometric air/fuel ratio due to the oxygen storage/release capability of the upstream-side catalyst 43. However, there occurs a case where the lean period is long despite the oxygen storage/release capability of the upstream-side catalyst 43. That case can be said to be the case where a large amount of oxygen that cannot be stored by the upstream-side catalyst 43 is flowing into the upstream-side catalyst 43, that is, the case where the air/fuel ratio of the mixture is leaner by a great extent than the stoichiometric air/fuel ratio. Therefore, in this embodiment, when it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture has switched from the rich side to the lean side of the stoichiometric air/fuel ratio, the stepwise increase value is made larger the longer the lean period.

On the other hand, it can be said that the longer the period during which the air/fuel ratio on the rich side of the stoichiometric air/fuel ratio is detected by the downstream-side air/fuel ratio sensor 56 (hereinafter, referred to as “rich period”), the greater the extent by which the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio. Specifically, the air/fuel ratio of the exhaust gas that flows out of the upstream-side catalyst 43 is considered to be equal to the stoichiometric air/fuel ratio due to the oxygen storage/release capability of the upstream-side catalyst 43. However, there occurs a case where the rich period is long despite the oxygen storage/release capability of the upstream-side catalyst 43. That case can be said to be the case where the amount of oxygen flowing into the upstream-side catalyst 43 is so small that the entire amount of oxygen stored in the upstream-side catalyst 43 is released, that is, the case where the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio by a great extent. Therefore, in this embodiment, when it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture has switched from the lean side to the rich side of the stoichiometric air/fuel ratio, the stepwise reduction value is made larger the longer the rich period.

By controlling the fuel injection amount in this manner, the air/fuel ratio of the mixture is accurately controlled to the stoichiometric air/fuel ratio as a whole.

Next, an example of a flowchart for executing a control of the fuel injection amount in accordance with this embodiment will be described. As the flowchart for executing the control of the fuel injection amount in accordance with this embodiment, flowcharts shown in FIGS. 5 to 7 can be used.

FIG. 5 shows a flowchart for calculating the time during which fuel is injected from each fuel injection valve 25. Upon start of the routine shown in FIG. 5, firstly the proportion Ga/N of the intake gas amount Ga to the engine rotation speed N is calculated in step 10. Next in step 11, a value Ga/N·α obtained by multiplying the proportion Ga/N calculated in step 10 by a constant α is input as a basic fuel injection duration TAUP. Next in step 12, a value TAUP·FAF·β·γ obtained by multiplying the basic fuel injection duration TAUP calculated in step 11 by an air/fuel ratio correction coefficient FAF (that is a coefficient calculated by a routine shown in FIG. 6, and will be described in detail later) as well as a constant β and α constant γ that are determined according to the state of operation of the internal combustion engine is input as a fuel injection duration TAU. After that, the routine ends. In this embodiment, fuel is injected from the fuel injection valves 25 for the fuel injection duration TAU calculated in step 12.

FIG. 6 is a flowchart for calculating the air/fuel ratio correction coefficient FAF that is for use in step 12 in FIG. 5. When the routine shown in FIG. 6 starts, firstly in step 20 it is determined whether the air/fuel ratio A/F detected by the upstream-side air/fuel ratio sensor 55 is greater than the stoichiometric air/fuel ratio A/Fst (A/F>A/Fst), that is, whether the air/fuel ratio of the exhaust gas discharged from the combustion chamber 21 is leaner than the stoichiometric air/fuel ratio. If it is determined that A/F>A/Fst, the routine proceeds to step 21 and subsequent steps. On the other hand, if it is determined that A/FsA/Fst, the routine proceeds to step 25 and subsequent steps.

If it is determined in step 20 that A/F>A/Fst, that is, if it is determined that the air/fuel ratio of the exhaust gas discharged from the combustion chamber 21 is leaner than the stoichiometric air/fuel ratio, it is then determined in step 21 whether the air/fuel ratio of the exhaust gas detected by the upstream-side air/fuel ratio sensor 55 has just switched from the rich side to the lean side of the stoichiometric air/fuel ratio. If it is determined in step 21 that the air/fuel ratio of the exhaust gas has just switched from the rich side to the lean side of the stoichiometric air/fuel ratio, the routine proceeds to step 22, in which a value FAF+RSR obtained by adding a stepwise increase value RSR (that is a value calculated by the routine shown in FIG. 7, and will be described in detail later) to the air/fuel ratio correction coefficient FAF calculated in the previous execution of the routine of FIG. 6 is set as a new air/fuel ratio correction coefficient FAF. Next in step 23, guarding is performed so that the air/fuel ratio correction coefficient FAF becomes a value within a permissible range. After that, the routine ends. On the other hand, if in step 21 it is determined that the air/fuel ratio of exhaust gas has not just switched from the rich side to the lean side of the stoichiometric air/fuel ratio, the routine proceeds to step 24, in which a value FAF+KIR obtained by adding a constant value KIR to the air/fuel ratio correction coefficient FAF calculated in the previous execution of the routine of FIG. 6 is set as a new air/fuel ratio correction coefficient FAF. Next in step 23, guarding is performed so that the air/fuel ratio correction coefficient FAF calculated in step 24 becomes a value within the permissible range. After that, the routine ends.

If in step 20 it is determined that A/FsA/Fst, that is, it is determined that the air/fuel ratio of the exhaust gas discharged from the combustion chamber 21 is richer than the stoichiometric air/fuel ratio, the routine proceeds to step 25 as mentioned above. In step 25, it is determined whether the air/fuel ratio of the exhaust gas detected by the upstream-side air/fuel ratio sensor 55 has just switched from the lean side to the rich side of the stoichiometric air/fuel ratio. If it is determined in step 25 that the air/fuel ratio of the exhaust gas has just switched from the lean side to the rich side of the stoichiometric air/fuel ratio, the routine proceeds to step 26, in which a value FAF-RSL obtained by subtracting a stepwise reduction value RSL (that is a value calculated in the routine shown in FIG. 7, and will be described in detail later) from the air/fuel ratio correction coefficient FAF calculated in the previous execution of the routine of FIG. 6 is set as a new air/fuel ratio correction coefficient FAF. Next in step 23, guarding is performed so that the air/fuel ratio correction coefficient FAF calculated in step 26 becomes a value within the permissible range. After that, the routine ends. On the other hand, if in step 25 it is determined that the air/fuel ratio of the exhaust gas has not just switched from the lean side to the rich side of the stoichiometric air/fuel ratio, the routine proceeds to step 27, in which a value obtained by subtracting a constant value KIL from the air/fuel ratio correction coefficient FAF calculated in the previous execution of the routine of FIG. 6 is set as a new air/fuel ratio correction coefficient. Next in step 23, guarding is performed so that the air/fuel ratio correction coefficient FAF calculated in step 27 becomes a value within the permissible range. After that, the routine ends.

FIG. 7 is a flowchart for calculating the stepwise increase value RSR for use in step 22 in FIG. 6 and the stepwise reduction value RSL for use in step 26 in FIG. 6. When the routine shown in FIG. 7 starts, it is firstly determined in step 30 whether the air/fuel ratio A/F of the exhaust gas detected by the downstream-side air/fuel ratio sensor 56 is greater than the stoichiometric air/fuel ratio A/Fst (A/F>A/Fst), that is, whether the air/fuel ratio of the exhaust gas flowing out of the upstream-side catalyst 43 is leaner than the stoichiometric air/fuel ratio. If it is determined in step 40 that A/F>A/Fst, the routine proceeds to step 31. On the other hand, if it is determined that A/F≦A/Fst, the routine proceeds to step 34.

If in step 30 it is determined that A/F>A/Fst, that is, if it is determined that the air/fuel ratio of the exhaust gas flowing out of the upstream-side catalyst 43 is leaner than the stoichiometric air/fuel ratio, then in step 31 a value RSR+ΔRS obtained by adding a predetermined amount ΔRS to the stepwise increase value RSR calculated in the previous execution of the routine of FIG. 7 is set as a new stepwise increase value RSR. Next in step 32, guarding is performed so that the stepwise increase value RSR calculated in step 31 becomes a value within a permissible value. Next in step 33, a value obtained by subtracting the stepwise increase value RSR guarded in step 32 from a constant R is set as a new stepwise reduction value RSL. After that, the routine ends.

On the other hand, if in step 30 it is determined that A/F≦A/Fst, that is, if it is determined that the air/fuel ratio of the exhaust gas flowing out of the upstream-side catalyst 43 is richer than the stoichiometric air/fuel ratio, the routine proceeds to step 34 as mentioned above. In step 34, a value RSR−ΔRS obtained by subtracting the predetermined value ΔRS from the stepwise increase value RSR calculated in the previous execution of the routine of FIG. 7 is set as a new stepwise increase value RSR. Next in step 32, guarding is performed so that the stepwise increase value RSR calculated in step 34 becomes a value within the permissible range. Next in step 33, a value obtained by subtracting the stepwise increase value RSR guarded in step 32 from the constant value R is set as a new stepwise increase value RSL. After that, the routine ends.

The internal combustion engine 10 has four fuel injection valves 25. If, of the fuel injection valves 25, for example, one fuel injection valve 25, has a fault, the following phenomenon occurs.

In the embodiment, the amount of fuel injected from each fuel injection valve 25 is controlled so that the air/fuel ratio of the mixture becomes equal to the target air/fuel ratio, on the basis of the air/fuel ratios of the exhaust gas detected by the air/fuel ratio sensors 55 and 56. That is, if it is determined that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio on the basis of the air/fuel ratios of the exhaust gas detected by the air/fuel ratio sensors 55 and 56, the fuel injection amount of each fuel injection valve 25 is increased. If it is determined that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio on the basis of the air/fuel ratios of the exhaust gas detected by air/fuel ratio sensors 55 and 56, the fuel injection amount of each fuel injection valve 25 is reduced. In other words, in this embodiment, the air/fuel ratio sensors 55 and 56 are provided not separately for each combustion chamber 21, but commonly for all the combustion chambers 25. Therefore, when it is determined that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio, it means that it is determined that, in all the combustion chambers 21, the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio. Likewise, when it is determined that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio, it means that it is determined that, in all the combustion chambers 21, the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio. Therefore, if it is determined that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio, the fuel injection amount is increased with respect to all the fuel injection valves 25. Likewise, if it is determined that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio, the fuel injection amount is reduced with respect to all the fuel injection valves 25.

For example, in the case where one of the fuel injection valves 25 has a fault of injecting an amount of fuel that is larger than the amount commanded by the electronic control unit 60 (hereinafter, referred to as “command value of fuel injection amount”) when the electronic control unit 60 outputs a command to each fuel injection valve 25 such that all the fuel injection valves 25 inject equal amounts of fuel (hereinafter, the fuel injection valve with this fault will be referred to as “abnormal fuel injection valve”), the air/fuel ratio of the mixture formed in the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 becomes richer than the stoichiometric air/fuel ratio when the other fuel injection valves (hereinafter, referred to as “normal fuel injection valves”) 25 inject the command value of injection amount of fuel and the air/fuel ratio of the mixture formed in each of the corresponding combustion chambers 21 is equal to the stoichiometric air/fuel ratio. Therefore, at this time, the emission level of the exhaust gas discharged from the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 deteriorates.

Then, when the exhaust gas discharged from the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 reaches the upstream-side air/fuel ratio sensor 55, it is then determined that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio, so that the fuel injection amount is reduced with respect to all the fuel injection valves 25. Hence, the air/fuel ratio of the mixture formed in each of the combustion chambers 21 that correspond to the normal fuel injection valves 25 becomes leaner than the stoichiometric air/fuel ratio. Therefore, at this time, the emission level of the exhaust gas discharged from the combustion chambers 21 that correspond to the normal fuel injection valves 25 also deteriorates.

Of course, although the air/fuel ratio of the mixture formed in the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 occasionally becomes richer than the stoichiometric air/fuel ratio and the air/fuel ratio of the mixture formed in each of the combustion chambers 21 that correspond to the normal fuel injection valves 25 occasionally becomes leaner than the stoichiometric air/fuel ratio, the air/fuel ratio control of this embodiment controls the fuel injection amount of the fuel injection valves 25 so that the air/fuel ratio of the mixture formed in the combustion chambers 21 will become equal to the stoichiometric air/fuel ratio. Therefore, according to this embodiment, it can be said that the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio as a whole. However, although the air/fuel ratio of the mixture can be said to be controlled to the stoichiometric air/fuel ratio as a whole, it is true, in view of the air/fuel ratio of the mixture formed in each one of the combustion chambers 21, that the air/fuel ratio of the mixture formed in a combustion chamber 21 occasionally becomes considerably richer than the stoichiometric air/fuel ratio and the air/fuel ratio of the mixture formed in another combustion chamber 21 occasionally becomes considerably leaner than the stoichiometric air/fuel ratio during execution of the air/fuel ratio control of this embodiment, and therefore that the emission level of the exhaust gas discharged from each combustion chamber 21 deteriorates during the execution of the control.

On another hand, in the case where one of the fuel injection valves 25 has a fault of injecting an amount of fuel that is smaller than the amount commanded by the electronic control unit 60, that is, the command value of fuel injection amount, when the electronic control unit 60 outputs to each fuel injection valve 25 a command such that all the fuel injection valves 25 inject equal amounts of fuel (hereinafter, the fuel injection valve with this fault will be also referred to as “abnormal fuel injection valve”), the air/fuel ratio of the mixture formed in the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 becomes leaner than the stoichiometric air/fuel ratio even when the normal fuel injection valves 25 inject the command value of injection amount of fuel and the air/fuel ratio of the mixture formed in the corresponding combustion chambers 21 is equal to the stoichiometric air/fuel ratio. Therefore, at this time, the emission level of the exhaust gas discharged from the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 deteriorates.

Then, as the exhaust gas discharged from the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 reaches the upstream-side air/fuel ratio sensor 55, it is determined that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio, and therefore the fuel injection amount is increased with respect to all the fuel injection valves 25. Hence, the air/fuel ratio of the mixture formed in each of the combustion chambers 21 that correspond to the normal fuel injection valves 25 becomes richer than the stoichiometric air/fuel ratio. Therefore, at this time, the emission level of the exhaust gas discharged from the combustion chambers that correspond to the normal fuel injection valves 25 also deteriorates.

Of course, although the air/fuel ratio of the mixture formed in the combustion chamber 21 that corresponds to the abnormal fuel injection valve 25 occasionally becomes leaner than the stoichiometric air/fuel ratio and the air/fuel ratio of the mixture formed in each of the combustion chambers 21 that correspond to the normal fuel injection valves 25 occasionally becomes richer than the stoichiometric air/fuel ratio, the air/fuel ratio control of this embodiment controls the fuel injection amount of the fuel injection valves 25 so that the air/fuel ratio of the mixture formed in the combustion chambers 21 will become equal to the stoichiometric air/fuel ratio. Therefore, it can be said that the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio as a whole. However, although the air/fuel ratio of the mixture can be said to be controlled to the stoichiometric air/fuel ratio as a whole, it is true, in view of the air/fuel ratio of the mixture formed in each one of the combustion chambers 21, that the air/fuel ratio of the mixture formed in a combustion chamber 21 occasionally becomes considerably leaner than the stoichiometric air/fuel ratio and the air/fuel ratio of the mixture formed in another combustion chamber 21 occasionally becomes considerably richer than the stoichiometric air/fuel ratio during execution of the air/fuel ratio control of this embodiment, and therefore that the emission level of the exhaust gas discharged from each combustion chamber 21 deteriorates during the execution of the control.

Thus, both in the case where a fuel injection valve 25 has a fault of injecting an amount of fuel larger than the command value of fuel injection amount and in the case where a fuel injection valve 25 has a fault of injecting an amount of fuel smaller than the command value of fuel injection amount, the emission level of the exhaust gas discharged from the combustion chambers 21 deteriorates.

Considering the foregoing circumstances, it is very important to know whether there is a state in which a certain fuel injection valve 25 has a fault such that the fuel injection valve 25 injects an amount of fuel larger than the command value of fuel injection amount or such that the fuel injection valve 25 injects an amount of fuel smaller than the command value of fuel injection amount, that is, whether there is a state in which there is a difference in the air/fuel ratio of the mixture among the combustion chambers (hereinafter, referred to as “state of inter-cylinder air/fuel ratio imbalance”), in order to know the state of emission of exhaust gas and take a countermeasure for eliminating or reducing the deterioration of the emission level of exhaust gas.

Therefore, in this embodiment, it is determined whether the state of inter-cylinder air/fuel ratio imbalance is present, that is, the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, on the basis of a finding as described below.

That is, the internal combustion engine 10 is designed so that the combustion chambers 21 of the cylinders undergo the exhaust stroke at timings spaced from each there by 180° in the crank angle, that is, the rotation angle of the crankshaft, sequentially in the order of the first cylinder #1, the fourth cylinder #4, the third cylinder #3 and the second cylinder #2. Therefore, exhaust gas is discharged sequentially from the combustion chambers 21 at intervals of 180° in the crank angle, and the exhaust gases from the combustion chambers 21 sequentially reach the upstream-side air/fuel ratio sensor 55. Therefore, generally the upstream-side air/fuel ratio sensor 55 sequentially detects the air/fuel ratio of the exhaust gas discharged from the first cylinder #1, the air/fuel ratio of the exhaust gas discharged from the fourth cylinder #4, the air/fuel ratio of the exhaust gas discharged from the third cylinder #3, and then the air/fuel ratio of the exhaust gas discharged from the second cylinder #2.

In the case where all the fuel injection valves 25 are normal, the output value that the upstream-side air/fuel ratio sensor 55 outputs corresponding to the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 (hereinafter, the output value will be referred to as “upstream-side air/fuel ratio sensor output value”) changes as shown in FIG. 8A. That is, as described above, according to the air/fuel ratio control of this embodiment, in the case where the air/fuel ratio of the exhaust gas formed in each combustion chamber 21 is to be controlled to the stoichiometric air/fuel ratio, the air/fuel ratio of the mixture formed in the combustion chamber 21 is made richer than the stoichiometric air/fuel ratio at some times and is made leaner than the stoichiometric air/fuel ratio at some other times so that the air/fuel ratio is controlled to the stoichiometric air/fuel ratio as a whole. Specifically, the following arrangement is provided. When it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture is leaner than the stoichiometric air/fuel ratio, the value of increase for the fuel injection amount of the fuel injection valves 25 is set so that the air/fuel ratio of the mixture reaches the stoichiometric air/fuel ratio as quickly as possible. When it is detected by the upstream-side air/fuel ratio sensor 55 that the air/fuel ratio of the mixture is richer than the stoichiometric air/fuel ratio, the value of reduction for the fuel injection amount of the fuel injection valves 25 is set so that the air/fuel ratio of the mixture reaches the stoichiometric air/fuel ratio as quickly as possible. Therefore, if all the fuel injection valves 25 are normal, the upstream-side air/fuel ratio sensor output value repeatedly increases and decreases in relatively small ranges on both side of an upstream-side air/fuel ratio sensor output value that corresponds to the stoichiometric air/fuel ratio as shown in FIG. 8A.

On another hand, in the case where the fuel injection valve 25 corresponding to the first cylinder #1 has a fault of injecting an amount of fuel that is larger than the command value of fuel injection amount while the fuel injection valves 25 corresponding to the other cylinders #2 to #4 are normal, the upstream-side air/fuel ratio sensor output value changes as shown in FIG. 8B. That is, since the air/fuel ratio of the mixture formed in the first cylinder #1 corresponding to the abnormal fuel injection valve 25 is considerably richer than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas discharged from the first cylinder #1 is also considerably richer than the stoichiometric air/fuel ratio. Therefore, when the exhaust gas discharged from the first cylinder #1 reaches the upstream-side air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor output value rapidly lessens toward an output value that corresponds to the air/fuel ratio of the exhaust gas discharged from the first cylinder #1, that is, an air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio. Then, according to the air/fuel ratio control of this control, when the upstream-side air/fuel ratio sensor output value reaches an output value that corresponds to the air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio, that is, when the upstream-side air/fuel ratio sensor 55 detects the air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio, the fuel injection amounts of all the fuel injection valves 25 are considerably reduced, so that the air/fuel ratio of the mixture formed in each of the fourth cylinder #4, the third cylinder #3 and the second cylinder #2 changes to an air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio. Therefore, when the exhaust gases discharged from the fourth cylinder #4, the third cylinder #3 and the second cylinder #2 reach the upstream-side air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor output value rapidly increases toward an output value that corresponds to the air/fuel ratio of the exhaust gas discharged from the cylinders #4, #3 and #2, that is, the air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio. Then, according to the air/fuel ratio control of this embodiment, when the upstream-side air/fuel ratio sensor output value reaches an output value that corresponds to the air/fuel ratio that is leaner than the stoichiometric air/fuel ratio, that is, when the upstream-side air/fuel ratio sensor 55 detects the air/fuel ratio that is leaner than the stoichiometric air/fuel ratio, the fuel injection amounts of all the fuel injection valves 25 are increased, so that the air/fuel ratio of the mixture formed in the first cylinder #1 corresponding to the abnormal fuel injection valve 25 changes again to an air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio. Therefore, in the case where a certain fuel injection valve 25 has a fault of injecting an amount of fuel that is larger than the command value of fuel injection amount, the upstream-side air/fuel ratio sensor output value repeatedly increases and decreases in relatively large ranges on both side of the output value that corresponds to the stoichiometric air/fuel ratio as shown in FIG. 8B.

On still another hand, in the case where the fuel injection valve 25 corresponding to the first cylinder #1 has a fault of injecting an amount of fuel that is smaller than the command value of fuel injection amount and where the fuel injection valves 25 corresponding to the other cylinder #2 to #4 are normal, the upstream-side air/fuel ratio sensor output value changes as shown in FIG. 8C. That is, since the air/fuel ratio of the mixture formed in the first cylinder #1 corresponding to the abnormal fuel injection valve 25 is considerably leaner than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas discharged from the first cylinder #1 is also considerably leaner than the stoichiometric air/fuel ratio. Therefore, when the exhaust gas discharged from the first cylinder #1 reaches the upstream-side air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor output value rapidly increases toward an output value that corresponds to the air/fuel ratio of the exhaust gas discharged from the first cylinder #1, that is, an air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio. Then, according to the air/fuel ratio control of this embodiment, when the upstream-side air/fuel ratio sensor output value reaches the output value that corresponds to the air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio, that is, when the upstream-side air/fuel ratio sensor 55 detects an air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio, the fuel injection amounts of all the fuel injection valves 25 are considerably increased, so that the air/fuel ratio of the mixture formed in each of the fourth cylinder #4, the third cylinder #3 and the second cylinder #2 changes to an air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio. Therefore, when the exhaust gases discharged from the fourth cylinder #4, the third cylinder #3 and the second cylinder #2 reach the upstream-side air/fuel ratio sensor 55, the upstream-side air/fuel ratio sensor output value rapidly lessens toward an output value that corresponds to the air/fuel ratio of the exhaust gas discharged from the cylinders #4 to #2, that is, an air/fuel ratio that is considerably richer than the stoichiometric air/fuel ratio. Then, according to the air/fuel ratio control of this embodiment, when the upstream-side air/fuel ratio sensor output value reaches an output value that corresponds to the air/fuel ratio that is richer than the stoichiometric air/fuel ratio, that is, when the upstream-side air/fuel ratio sensor 55 detects the air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the fuel injection amounts of all the fuel injection valves 25 are reduced, so that the air/fuel ratio of the mixture formed in the first cylinder #1 changes again to an air/fuel ratio that is considerably leaner than the stoichiometric air/fuel ratio. Therefore, in the case where a certain fuel injection valve 25 has a fault of injecting an amount of fuel that is larger than the command value of fuel injection amount, the upstream-side air/fuel ratio sensor output value repeatedly increases and decreases in relatively large ranges on both side of the output value that corresponds to the stoichiometric air/fuel ratio, as shown in FIG. 8C.

Thus, the transition of the upstream-side air/fuel ratio sensor output value occurring when a certain fuel injection valve 25 has abnormality is greatly different from the transition of the upstream-side air/fuel ratio sensor output value occurring when all the fuel injection valves 25 are normal.

In particular, in the case where all the fuel injection valves 25 are normal, the average slope of a line that the upstream-side air/fuel ratio sensor output value follows (hereinafter, the average slope will be referred to simply as “slope”) is a slope α1 that is relatively small in absolute value when the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, as shown in FIG. 8A. On the other hand, when the upstream-side air/fuel ratio sensor output value increases as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side, the average slope of the line that the upstream-side air/fuel ratio sensor output value follows (hereinafter, this average slope will also be referred to simply as “slope”) is a relatively small slope α2. In this case, the absolute value of the slope α1 and the absolute value of the slope α2 are substantially equal.

In the case where a certain fuel injection valve 25 has a fault of injecting an amount of fuel that is larger than the command value of fuel injection amount, the average slope of a line that the upstream-side air/fuel ratio sensor output value follows is a slope α3 that is relatively large in absolute value when the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, as shown in FIG. 8B. On the other hand, when the upstream-side air/fuel ratio sensor output value increases as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side, the average slope of the line that the upstream-side air/fuel ratio sensor output value follows is a relatively large slope α4. In this case, the absolute value of the slope α3 of the line that the upstream-side air/fuel ratio sensor output value follows when the upstream-side air/fuel ratio sensor output value lessens is slightly larger than the absolute value of the slope α4 of the line that the upstream-side air/fuel ratio sensor output value follows when the upstream-side air/fuel ratio sensor output value increases.

In the case where a certain fuel injection valve 25 has a fault of injecting an amount of fuel that is smaller than the command value of fuel injection amount, the average slope of a line that the upstream-side air/fuel ratio sensor output value follows is a relatively large slope α5 when the upstream-side air/fuel ratio sensor output value increases as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side, as shown in FIG. 8C. On the other hand, when the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, the average slope of the line that the upstream-side air/fuel ratio sensor output value follows is a slope α6 that is relatively large in absolute value. In this case, the absolute value of the slope α5 of the line that the upstream-side air/fuel ratio sensor output value follows when the upstream-side air/fuel ratio sensor output value increases is slightly larger than the absolute value of the slope α6 of the line that the upstream-side air/fuel ratio sensor output value follows when the upstream-side air/fuel ratio sensor output value lessens.

Thus, the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows assumes different characteristic values in the case where all the fuel injection valves 25 are normal, the case where a certain fuel injection valve 25 has an abnormality of injecting an amount of fuel that is larger than the command value of fuel injection amount, and the case where a certain fuel injection valve 25 has an abnormality of injecting an amount of fuel that is smaller than the command value of fuel injection amount fuel. Therefore, by using the absolute value of the slope, it is possible to determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance. Specifically, the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows when a certain fuel injection valve 25 has abnormality is basically larger than the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows when all the fuel injection valves 25 are normal. Therefore, if the absolute value of a slope that can be assumed by the line that upstream-side air/fuel ratio sensor output value follows in the case where all the fuel injection valves 25 are normal is set as a threshold value, or if a value that is larger than that absolute value of the slope is set as a threshold value, it can be determined that the state of inter-cylinder air/fuel ratio imbalance is present when the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows is larger than the threshold value during operation of the engine.

By the way, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance described above with reference to FIG. 8A to FIG. 8C is performed in the case where the air/fuel ratio of the mixture is to be controlled to the stoichiometric air/fuel ratio (i.e., the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is the stoichiometric air/fuel ratio). However, this determination of the presence or absence of the state of air/fuel ratio imbalance can also be applied to the case where the air/fuel ratio of the mixture is to be controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio (i.e., the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is richer than the stoichiometric air/fuel ratio).

That is, in the case where all the fuel injection valves 25 are normal when the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor output value (i.e., the output value of the upstream-side air/fuel ratio sensor 55) changes as shown in FIG. 9A, corresponding to the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55. Specifically, as for the transition of the upstream-side air/fuel ratio sensor output value at this time, the output value repeatedly increases and decreases in relatively small ranges, and the entire curve indicating the transition of the output value is positioned in the richer air/fuel ratio side in comparison with the transition of the upstream-side air/fuel ratio sensor output value shown in FIG. 8A. However, when the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, an average slope of a line shown in FIG. 9A that the upstream-side air/fuel ratio sensor output value follows is the slope α1 relatively small in absolute value, as in the case of the average slope of the line shown in FIG. 8A that the upstream-side air/fuel ratio sensor output value follows. Likewise, when the upstream-side air/fuel ratio sensor output value increases as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side, an average slope of the line shown in FIG. 9A that the upstream-side air/fuel ratio sensor output value follows is the relatively small slope α2, as in the case of the average slope of the line shown in FIG. 8A that the upstream-side air/fuel ratio sensor output value follows.

Furthermore, in the case where the fuel injection valve 25 corresponding to the first cylinder #1 has a fault of injecting an amount of fuel that is larger than the command value of fuel injection amount (i.e., the amount of fuel that the electronic control unit 60 commands each fuel injection valve 25 to inject) and the fuel injection valves 25 corresponding to the other cylinders #2 to #4 are normal when the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor output value changes as shown in FIG. 9B. As for the transition of the upstream-side air/fuel ratio sensor output value at this time, the output value repeatedly increases and decreases in relatively large ranges, and the entire curve indicating the transition of the output value is positioned in the richer air/fuel ratio side in comparison with the transition of the upstream-side air/fuel ratio sensor output value shown in FIG. 8B. When the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, an average slope of a line shown in FIG. 9B that the upstream-side air/fuel ratio sensor output value follows is the slope α3 relatively large in absolute value, as in the case of the average slope of the line shown in FIG. 8B that the upstream-side air/fuel ratio sensor output value follows. Likewise, an average slope of the line that the upstream-side air/fuel ratio sensor output value follows as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side is the relatively large slope α4, as in the case of the average slope of the line that the upstream-side air/fuel ratio sensor output shown in FIG. 8B follows.

Furthermore, in the case where the fuel injection valve 25 corresponding to the first cylinder #1 has a fault of injecting an amount of fuel that is smaller than the command value of fuel injection amount and the fuel injection valves 25 corresponding to the other cylinders #2 to #4 are normal when the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor output value changes as shown in FIG. 9C. As for the transition of the upstream-side air/fuel ratio sensor output value, the output value repeatedly increases and decreases in relatively large ranges, and the entire curve indicating the transition of the output value is positioned in the richer air/fuel ratio side, in comparison with the transition of the upstream-side air/fuel ratio sensor output value shown in FIG. 8C. When the upstream-side air/fuel ratio sensor output value increases as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the leaner side, an average slope of a line shown in FIG. 9C that the upstream-side air/fuel ratio sensor output value follows is the relatively large slope α5, as in the case of the average slope of the line shown in FIG. 8C that the upstream-side air/fuel ratio sensor output value follows. Likewise, when the upstream-side air/fuel ratio sensor output value lessens as the air/fuel ratio of the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 changes to the richer side, an average slope of the line shown in FIG. 9C that the upstream-side air/fuel ratio sensor output value follows is the slope α6 relatively large in absolute value, as in the case of the average slope of the line shown in FIG. 8C that the upstream-side air/fuel ratio sensor output value follows.

Thus, when the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio, too, the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows in the case where a certain fuel injection valve 25 has abnormality (i.e., where the state of inter-cylinder air/fuel ratio imbalance is present) is larger than the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows in the case where all the fuel injection valves 25 are normal (i.e., where the state of inter-cylinder air/fuel ratio imbalance is not present), similarly to when the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio. Therefore, if the absolute value of the slope that can be assumed by the line that the upstream-side air/fuel ratio sensor output value follows in the case where all the fuel injection valves 25 are normal when the air/fuel ratio of the mixture is controlled to an air/fuel ratio that is richer than the stoichiometric air/fuel ratio is set beforehand as a threshold value, or if a value that is larger than the absolute value of the slope is set beforehand as a threshold value, it can be determined that the state of inter-cylinder air/fuel ratio imbalance is present when the absolute value of the slope of the line that the upstream-side air/fuel ratio sensor output value follows when the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio is larger than the threshold value.

By the way, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance described above with reference to FIG. 8A to FIG. 8C is performed in the case where the air/fuel ratio of the mixture is to be controlled to the stoichiometric air/fuel ratio (i.e., the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is the stoichiometric air/fuel ratio). However, this determination of the presence or absence of the state of air/fuel ratio imbalance can also be applied to the case where the air/fuel ratio of the mixture is to be controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio (i.e., the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is leaner than the stoichiometric air/fuel ratio), just as the determination of the presence or absence of the state of air/fuel ratio imbalance can be applied to the case where the air/fuel ratio of the mixture is to be controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio.

Thus, the foregoing determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be performed in any one of the case where the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio, the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

However, in the case where the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio and the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance based on the foregoing idea is lower than in the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio. A reason why the accuracy of the determination is low when the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined in the case where the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio and the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio will be explained. Besides, a reason why the determination accuracy is high when the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined in the case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio will also be explained.

Firstly, a reason why the determination accuracy is low when the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined in the case where the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio will be explained.

As described above, the upstream-side air/fuel ratio sensor 55 outputs an output value that follows the characteristic shown in FIG. 3A, according to the air/fuel ratio of the exhaust gas that comes to the sensor 55. A mechanism by which the upstream-side air/fuel ratio sensor 55 outputs the output value in this manner is as follows.

That is, the upstream-side air/fuel ratio sensor 55, as shown in FIG. 10 and FIG. 11, has an air/fuel ratio detection element 55 a, an outer protection cover 55 b, and an inner protection cover 55 c. The protection covers 55 b and 55 c house, inside thereof, the air/fuel ratio detection element 55 a so as to cover the air/fuel ratio detection element 55 a. Besides, the protection covers 55 b and 55 c have inflow holes 55 b 1 and 55 c 1 that allow the exhaust gas that reaches the upstream-side air/fuel ratio sensor 55 to flow from the exhaust pipe 42 into an internal space inside the protection covers 55 b and 55 c and reach the air/fuel ratio detection element 55 a, and outflow holes 55 b 2 and 55 c 2 that allow the exhaust gas that flows into the internal space to flow out into the exhaust pipe 42.

The upstream-side air/fuel ratio sensor 55 is disposed on the exhaust pipe 42 so that the protection covers 55 b and 55 c are exposed to an internal space of the exhaust pipe 42. Therefore, exhaust gas EX that flows in the exhaust pipe 42 flows into a space between the outer protection cover 55 b and the inner protection cover 55 c through the inflow hole 55 b 1 of the outer protection cover 55 b, as shown by an arrow Ar1 in FIG. 10 and FIG. 11. Next, the exhaust gas flows into an internal space of the inner protection cover 55 c through the inflow hole 55 c 1 of the inner protection cover 55 c, and reaches the air/fuel ratio detection element 55 a, as shown by an arrow Ar2. After that, the exhaust gas flows out into the exhaust pipe 42 through the outflow hole 55 c 2 of the inner protection cover 55 c and the outflow hole 55 b 2 of the outer protection cover 55 b, as shown by an arrow Ar3. Since the exhaust gas having reached the upstream-side air/fuel ratio sensor 55 flows in the upstream-side air/fuel ratio sensor 55 in this manner, the exhaust gas having reached the upstream-side air/fuel ratio sensor 55 is drawn into the inflow hole 55 b 1 of the outer protection cover 55 b, due to the flow of exhaust gas moving in the vicinity of the outflow hole 55 b 2 of the outer protection cover 55 b.

The air/fuel ratio detection element 55 a, as shown in FIG. 12A, has a solid electrolyte layer 551, an exhaust gas-side electrode layer 552, an atmosphere-side electrode layer 553, a diffusion resistance layer (or diffusion rate-determining layer) 554, an exhaust gas-side wall 555, an atmosphere-side wall 556, and a heater 557.

The solid electrolyte layer 551 is a sintered body of an oxygen ion-conductive oxide, for example, a stabilized zirconia element in which CaO as a stabilizer is dissolved in ZrO₂ (zirconia) in a solid form. The solid electrolyte layer 551 exerts an oxygen cell characteristic and an oxygen pump characteristic (that will be described later) when its temperature is higher than a certain temperature (i.e., higher than a so-called activation temperature).

Besides, the exhaust gas-side electrode layer 552 includes a noble metal that has high catalytic activity, for example, Pt (platinum). The exhaust gas-side electrode layer 552 is disposed on a surface of the solid electrolyte layer 551. Besides, the exhaust gas-side electrode layer 552 is formed, for example, by chemical plating, so as to have sufficient permeability, that is, so as to be porous.

On the other hand, the atmosphere-side electrode layer 553 includes a noble metal that has high catalytic activity, for example, Pt (platinum). Besides, the atmosphere-side electrode layer 553 is disposed on a surface of the solid electrolyte layer 551 that is opposite the surface thereof on which the electrode layer 552 is disposed. That is, the solid electrolyte layer 551 is sandwiched between the exhaust gas-side electrode layer 552 and the atmosphere-side electrode layer 553. Besides, the atmosphere-side electrode layer 553 is formed, for example, by chemical plating, so as to have sufficient permeability, that is, so as to be porous.

An electric power source 560 is connected to the exhaust gas-side electrode layer 552 and the atmosphere-side electrode layer 553. Voltage is applied from the electric power source 560 to the exhaust gas-side electrode layer 552 and the atmosphere-side electrode layer 553 so that the electric potential of the atmosphere-side electrode layer 553 is higher than that of the exhaust gas-side electrode layer 552.

The diffusion resistance layer 554 is made of a porous ceramic material that is a heat-resistant inorganic substance. The diffusion resistance layer 554 is disposed, for example, by a plasma spraying method, so as to cover the surfaces of the exhaust gas-side electrode layer 552 except the surface thereof that is in contact with the adjacent surface of the solid electrolyte layer 551.

The exhaust gas-side wall 555 is made of an alumina ceramics that is dense and impermeable to exhaust gas. Besides, the exhaust gas-side wall 555 is disposed so as to cover the diffusion resistance layer 554 except portions thereof (particularly, corner portions of the diffusion resistance layer 554). Specifically, the exhaust gas-side wall 555 has through holes 558 each of which exposes a portion of the diffusion resistance layer 554 to the outside.

The atmosphere-side wall 556 is made of an alumina ceramics that is dense and impermeable to exhaust gas. Besides, the atmosphere-side wall 556 is disposed so that the atmosphere-side wall 556 partially defines, on its inner side, a space 559 that surrounds the atmosphere-side electrode layer 553 (hereinafter, this space will be referred to as “atmospheric chamber”). Atmospheric air is introduced into the atmospheric chamber 559.

The heater 557 is buried in the atmosphere-side wall 556. When supplied with electric power, the heater 557 generates heat, so that the solid electrolyte layer 551, the exhaust gas-side electrode layer 552 and the atmosphere-side electrode layer 553 are heated.

The air/fuel ratio detection element 55 a functions as shown in FIG. 12B when an exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio comes to the air/fuel ratio detection element 55 a. That is, since the concentration of oxygen in exhaust gas around the exhaust gas-side electrode layer 552 is relatively high, oxygen moves from the exhaust gas-side electrode layer 552 to the atmosphere-side electrode layer 553 through the solid electrolyte layer 551. Specifically, exhaust gas that reaches the air/fuel ratio detection element 55 a flows into the diffusion resistance layer 554 through the through holes 558. Then, when the exhaust gas reaches the exhaust gas-side electrode layer 552 through the diffusion resistance layer 554, oxygen in the exhaust gas is ionized by the exhaust gas-side electrode layer 552. The ionized oxygen, that is, oxygen ions, reaches the atmosphere-side electrode layer 553 through the solid electrolyte layer 551. After reaching the atmosphere-side electrode layer 553, the oxygen ions lose electrons to the atmosphere-side electrode layer 553 and become oxygen, which in turn flows into the atmospheric chamber 559. Through this operation, current I flows from the positive electrode to the negative electrode of the electric power source 560. The magnitude of the current I that flows in this manner is a constant value that is proportional to the concentration of oxygen in the exhaust gas that reaches the exhaust gas-side electrode layer 552 if the voltage of the electric power source 560 is set at a predetermined value Vp as shown in FIG. 13. In this embodiment, when the voltage of the electric power source 560 is set at the predetermined value Vp and the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is leaner than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas is known on the basis of the current I (i.e., the limiting current Ip) that flows from the positive electrode toward the negative electrode of the electric power source 560.

On the other hand, when an exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio comes to the air/fuel ratio detection element 55 a, the air/fuel ratio detection element 55 a functions as shown in FIG. 12C. That is, since the concentration of oxygen in the exhaust gas around the exhaust gas-side electrode layer 552 is relatively low, oxygen moves from the atmosphere-side electrode layer 553 to the exhaust gas-side electrode layer 552 through the solid electrolyte layer 551. Specifically, oxygen in the atmospheric air introduced in the atmospheric chamber 559 is ionized by the atmosphere-side electrode layer 553. The ionized oxygen, that is, oxygen ions, reaches the exhaust gas-side electrode layer 553 through the solid electrolyte layer 551. After reaching the exhaust gas-side electrode layer 552, the oxygen ions lose electrons to the exhaust gas-side electrode layer 552 and oxidize unburnt materials in the exhaust gas around the exhaust gas-side electrode layer 552, for example, hydrocarbons (HCs), carbon monoxide (CO) and hydrogen (H₂). Through this operation, current I flows from the negative electrode to the positive electrode of the electric power source 560. The magnitude of the current I that flows in this manner is also a constant value that is proportional to the concentration of oxygen in the exhaust gas around the exhaust gas-side electrode layer 552 if the voltage of the electric power source 560 is set at the predetermined value Vp as shown in FIG. 13. In this embodiment, when the voltage of the electric power source 560 is set at the predetermined value Vp and the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is richer than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas is known on the basis of the current I (i.e., the limiting current Ip) that flows from the negative electrode to the positive electrode of the electric power source 560.

As described above, the upstream-side air/fuel ratio sensor 55 outputs an output value commensurate with the air/fuel ratio of the exhaust gas that comes to the sensor 55. That is, when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is leaner than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor 55 outputs a positive value of current that is greater the greater the degree of leanness of the air/fuel ratio of the exhaust gas that comes to the sensor 55. When the air/fuel ratio of the exhaust gas that comes to the sensor 55 is richer than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor 55 outputs a negative value of current whose absolute value is greater the greater the degree of richness of the air/fuel ratio of the exhaust gas that comes to the sensor 55. Therefore, when the air/fuel ratio of the exhaust gas that flows into the upstream-side air/fuel ratio sensor 55 is the stoichiometric air/fuel ratio, the value of the current that the upstream-side air/fuel ratio sensor 55 outputs is zero.

Therefore, in the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio leaner than the stoichiometric air/fuel ratio to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the output value of the upstream-side air/fuel ratio sensor 55 changes from a positive value of current to a negative value of current. In this case, oxygen ions flowing from the exhaust gas-side electrode layer 552 to the atmosphere-side electrode layer 553 of the upstream-side air/fuel ratio sensor 55 through the solid electrolyte layer 551 come to flow from the atmosphere-side electrode layer 553 to the exhaust gas-side electrode layer 552 through the solid electrolyte layer 551. That is, the flow direction of oxygen ions flowing in the solid electrolyte layer 551 reverses. However, when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio leaner than the stoichiometric air/fuel ratio to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the oxygen ions having been flowing in the solid electrolyte layer 551 from the exhaust gas-side electrode layer 552 toward the atmosphere-side electrode layer 553 need to changes the flow direction thereof. Therefore, when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from the lean side of the stoichiometric air/fuel ratio to the rich side of the stoichiometric air/fuel ratio, the direction of flow of oxygen ions in the solid electrolyte layer 551 does not instantly reverse.

On the other hand, in the case where the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the output value of the upstream-side air/fuel ratio sensor 55 changes from a negative value of current to a positive value of current. In this case, oxygen ions having been flowing from the atmosphere-side electrode layer 553 to the exhaust gas-side electrode layer 552 of the upstream-side air/fuel ratio sensor 55 through the solid electrolyte layer 551 come to flow from the exhaust gas-side electrode layer 552 to the atmosphere-side electrode layer 553 through the solid electrolyte layer 551. That is, the flow direction of oxygen ions flowing in the solid electrolyte layer 551 reverses. However, when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the oxygen ions having been flowing in the solid electrolyte layer 551 from the atmosphere-side electrode layer 553 toward the exhaust gas-side electrode layer 552 need to change the flow direction thereof. Therefore, when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes form an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the flow direction of oxygen ions in the solid electrolyte layer 551 does not instantly reverse.

It is to be noted herein that when the control of bringing the air/fuel ratio of the mixture to the stoichiometric air/fuel ratio is being performed, the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 repeatedly changes between the lean side of the stoichiometric air/fuel ratio and the rich side of the stoichiometric air/fuel ratio. Therefore, in this case, in order for the upstream-side air/fuel ratio sensor 55 to output an output value that very accurately shows the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55, the flow direction of oxygen ions flowing in the solid electrolyte layer 551 of the upstream-side air/fuel ratio sensor 55 needs to instantly reverse when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes between the lean side of the stoichiometric air/fuel ratio and the rich side of the stoichiometric air/fuel ratio, that is, from the lean side to the rich side or the other way around. However, as described above, the flow direction of oxygen flowing in the solid electrolyte layer 551 does not instantly reverse. Therefore, when the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio, the output value of the upstream-side air/fuel ratio sensor 55 does not very accurately show the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 (i.e., the air/fuel ratio of the mixture).

In order to accurately determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55, it is desired that the output value of the upstream-side air/fuel ratio sensor 55 very accurately show the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 (i.e., the air/fuel ratio of the mixture). In other words, when the control of bringing the air/fuel ratio of the mixture to the stoichiometric air/fuel ratio is being performed, the output value of the upstream-side air/fuel ratio sensor 55 does not very accurately show the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55, and therefore it is impossible to accurately determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55.

For the foregoing reason, the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance becomes low if the determination is performed while the control of bringing the air/fuel ratio of the mixture to the stoichiometric air/fuel ratio is being performed.

Next explained will be a reason why the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance becomes low if the determination is performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

When the air/fuel ratio of the mixture is being controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the combustion in each combustion chamber 21 tends to be unstable, and in some cases, misfire occurs in a combustion chamber 21. In such a case, the air/fuel ratio of the exhaust gas discharged from each combustion chamber 21 does not reflect the air/fuel ratio of the mixture that is to be achieved by the control.

As stated above, in order to accurately determine the presence or absence the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55, it is desired that the output value of the upstream-side air/fuel ratio sensor 55 very accurately show the air/fuel ratio of the mixture. However, when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas discharged from each combustion chamber 21 does not reflect the air/fuel ratio of the mixture that is to be achieved by the control as mentioned above, and therefore it is not possible to accurately determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55.

For the foregoing reason, the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance becomes low if the determination is performed when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

Next explained will be a reason why the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance becomes high if the determination is performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio.

When the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is also richer than the stoichiometric air/fuel ratio. Therefore, in this case, the upstream-side air/fuel ratio sensor 55 outputs an output value that accurately shows the air/fuel ratio of the exhaust gas that comes to the sensor 55, without a need for the reversal of the flow direction of oxygen ions flowing in the diffusion resistance layer 551 of the upstream-side air/fuel ratio sensor 55. That is, when the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the output value of the upstream-side air/fuel ratio sensor 55 very accurately shows the air/fuel ratio of the exhaust gas that comes to the sensor 55 (i.e., the air/fuel ratio of the mixture), and therefore it is possible to accurately determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55.

Besides, when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the combustion in each combustion chamber 21 is stable, and therefore the air/fuel ratio of the exhaust gas discharged from each combustion chamber 21 reflects the air/fuel ratio of the mixture that is to be achieved by the control. Therefore, this means that when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the upstream-side air/fuel ratio sensor 55 very accurately shows the air/fuel ratio of the mixture, and therefore that it is possible to accurately determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance on the basis of the output value of the upstream-side air/fuel ratio sensor 55.

For the foregoing reason, the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance becomes high if the determination is performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio.

By the way, as stated above, the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is high if the determination is performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio. Therefore, when the presence or absence of the state of inter-cylinder air/fuel ratio imbalance needs to be determined, the determination of the presence or absence thereof can be accurately performed by performing the determination while forcing the air/fuel ratio of the mixture to be an air/fuel ratio richer than the stoichiometric air/fuel ratio.

However, if the air/fuel ratio of the mixture is brought to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 deteriorates. Sometimes, the internal combustion engine 10 causes the air/fuel ratio of the mixture to be richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56. The determination of the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56 is performed as follows.

After the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21. Then, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43. At this time, since the upstream-side catalyst 43 has an oxygen storage/release capability, the oxygen stored in the upstream-side catalyst 43 is let out from the upstream-side catalyst 43, so that exhaust gas of the stoichiometric air/fuel ratio flows out from the upstream-side catalyst 43. Then, when all the oxygen stored in the upstream-side catalyst 43 is used as exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio continues to flow into the upstream-side catalyst 43, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio begins to flow out from the upstream-side catalyst 43. At this time, if the downstream-side air/fuel ratio sensor 56 does not have abnormality regarding its output, that is, if the downstream-side air/fuel ratio sensor 56 is normal, the output value of the downstream-side air/fuel ratio sensor 56 (hereinafter, referred to as “downstream-side air/fuel ratio sensor output value”) accurately corresponds to the air/fuel ratio of the exhaust gas that flows out from the upstream-side catalyst 43, that is, the air/fuel ratio of the mixture that is being controlled to the rich side of the stoichiometric air/fuel ratio. Therefore, if the downstream-side air/fuel ratio sensor output value is an output value that accurately corresponds to the air/fuel ratio of the mixture when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is flowing out from the upstream-side catalyst 43, it can be said that there is no abnormality of the output of the downstream-side air/fuel ratio sensor 56 and therefore the downstream-side air/fuel ratio sensor 56 is normal. On the other hand, if the downstream-side air/fuel ratio sensor output value is an output value that does not correspond to the air/fuel ratio of the mixture when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is flowing out from the upstream-side catalyst 43, it can be said that there is abnormality of the output of the downstream-side air/fuel ratio sensor 56.

Therefore, in the internal combustion engine 10, when the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56 needs to be determined, the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio over a predetermined period of time. The predetermined period of time herein is set to a time that is sufficiently long for the oxygen stored in the upstream-side catalyst 43 to run out if exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio continues to flow into the upstream-side catalyst 43. Then, when the foregoing predetermined period elapses after the air/fuel ratio of the mixture begins to be controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is determined whether the downstream-side air/fuel ratio sensor output value shows an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio. Then, if it is determined that the downstream-side air/fuel ratio sensor output value at this time shows an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is then determined that there is no abnormality of the output of the downstream-side air/fuel ratio sensor 56 and therefore the downstream-side air/fuel ratio sensor 56 is normal. On the other hand, if it is determined that the downstream-side air/fuel ratio sensor output value at this time does not show an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is then determined that there is abnormality of the output of the downstream-side air/fuel ratio sensor 56.

In this embodiment (hereinafter, referred to as “first embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the first embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the first embodiment utilizes flowcharts shown in FIG. 14 and FIG. 15. The routine shown in FIG. 14 and the routine shown in FIG. 15 are each executed at every predetermined time interval.

When the routine shown in FIG. 14 starts, it is firstly determined in step 100 whether a flag F1 that shows whether execution of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is permitted (hereinafter, referred to as “inter-cylinder air/fuel ratio imbalance determination execution flag F1”) has been set (F1=1). If the inter-cylinder air/fuel ratio imbalance determination execution flag F1 has been set (F1=1), it means that execution of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is permitted. If the inter-cylinder air/fuel ratio imbalance determination execution flag F1 has been reset (F1=0), it means that execution of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not permitted. Besides, the inter-cylinder air/fuel ratio imbalance determination execution flag F1 is set, for example, in accordance with the routine shown in FIG. 15.

If in step 100 it is determined that F1=1, it means that execution of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is permitted. Therefore, in order to execute the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the routine proceeds to step 101. On the other hand, if in step 100 it is determined that F1=0, it means that execution of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not permitted, and therefore the routine immediately ends.

In step 101, to which the routine proceeds after in step 100 it is determined that F1=1, the absolute value |ΔA/F| of the rate of change in the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is calculated on the basis of the output value of the upstream-side air/fuel ratio sensor 55. Next, in step 102, a counter value C that shows the number of times that the absolute value |ΔA/F| of the rate of change in the air/fuel ratio of the exhaust gas has been calculated in step 101 is incremented by one.

Next in step 103, it is determined whether the counter value C incremented by one in step 102 has become equal to a predetermined threshold value Cth (C=Cth). If it is determined that C=Cth, the routine proceeds to step 104. On the other hand, if it is determined that C≠Cth, the routine returns to step 100.

Incidentally, step 102 and step 103 are provided for acquiring an increased number of values of the rate of change in the air/fuel ratio of exhaust gas calculated in step 101 in order to heighten the accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance. Therefore, steps 102 and 103 may be omitted in the case where high accuracy of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is achieved even if the number of values of the rate of change in the air/fuel ratio of exhaust gas calculated in step 101 is one.

When the routine returns to step S100 after it is determined in step S103 that C≠Cth, it is determined again whether the inter-cylinder air/fuel ratio imbalance determination execution flag F1 has been set (F1=1). If in step S101 it is determined that F1=1, the routine proceeds to step 101 and the following steps. On the other hand, if it is determined that F1=0, the routine ends.

In step 104, to which the routine proceeds after in step 103 it is determined that C=Cth, an average value ΔA/Fave of the absolute values of the rate of change in the air/fuel ratio of exhaust gas calculated by a plurality of executions of step 101 is calculated.

Next in step 105, it is determined whether the average value ΔA/Fave calculated in step 104 is greater than a predetermined threshold ΔA/Faveth (ΔA/Fave>ΔA/Faveth). If it is determined that ΔA/Fave>ΔA/Faveth, it means that the state of inter-cylinder air/fuel ratio imbalance is present. Then, the routine proceeds to step 106, in which an alarm that shows that the state of inter-cylinder air/fuel ratio imbalance is present is activated.

Then, because the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has now ended, in step 107 the absolute values |ΔA/F| of the rate of change in the air/fuel ratio of exhaust gas calculated in step 101 are cleared. Subsequently in step 108, the counter value C incremented in step 102 is cleared. After that, the routine ends.

On another hand, if in step 105 it is determined that ΔA/Fave≦ΔA/Faveth, that it, if it is determined that the average value ΔA/Fave calculated in step 104 is less than or equal to the predetermined threshold value ΔA/Faveth, it means that the state of inter-cylinder air/fuel ratio imbalance is not present, and therefore the routine immediately ends.

Next described will be the routine shown by the flowchart in FIG. 15 that is an example of the routine of executing the setting of the inter-cylinder air/fuel ratio imbalance determination execution flag in the first embodiment which is utilized in step 100 in the routine shown in FIG. 14.

When the routine shown in FIG. 15 starts, firstly in step 200 it is determined whether a flag F2 that shows whether a condition that is a prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied (hereinafter, this flag will be referred to as “inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2”) has been set (F2=1). If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. On the other hand, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 200 it is determined that F2=1, it means that the condition as the prerequisite for executing determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and then routine proceeds to step 201. On the other hand, if it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and then the routine proceeds to step 203.

Incidentally, the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is a condition that the temperature of a coolant for cooling the internal combustion engine 10 (this temperature represents the temperature of the internal combustion engine 10) is higher than or equal to a predetermined temperature (e.g., 75° C.), and that the engine rotation speed is within a predetermined range (e.g., of 1200 rpm to 2000 rpm), and that the intake gas amount is within a predetermined range (e.g., of 10 g/sec to 20 g/sec), and that the temperature of the upstream-side air/fuel ratio sensor 55 is higher than or equal to its activation temperature, and that the atmospheric temperature is higher than or equal to a predetermined value (e.g., 75 kPa).

In step 201, to which routine proceeds after step 200 it is determined that F2=1, it is determined whether a flag F3 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56 (hereinafter, this flag will be referred to as “first abnormality determination-purpose rich air/fuel ratio control flag F3”) has been set (F3=1). If the first abnormality determination-purpose rich air/fuel ratio control flag F3 has been set (F3=1), it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56. If the first abnormality determination-purpose rich air/fuel ratio control flag F3 has been reset (F3=0), it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56.

If in step 201 it is determined that F3=1, it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56. Then, the routine proceeds to step S202, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 201 is when in step 200 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1) Therefore, when it is determined in step 201 that the first abnormality determination-purpose rich air/fuel ratio control flag F3 has been set (F3=1), the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, so that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 202, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 201 it is determined that F3=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality of the output of the downstream-side air/fuel ratio sensor 56. Then, the routine proceeds to step 203, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 201, it has been determined in step 200 that F2=1, which means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 201 it is determined that F3=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 203, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 200 it is determined that F2.0, the routine proceeds to step 203, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. Specifically, when in step 200 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 203, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, and therefore the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, in the internal combustion engine 10, sometimes the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43. The determination of the presence or absence of the oxygen storage/release capability of the upstream-side catalyst 43 is performed as follows.

That is, when the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21. Then, the exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43. Since the upstream-side catalyst 43 has the oxygen storage/release capability, the oxygen stored in the upstream-side catalyst 43 is discharged from the upstream-side catalyst 43, so that exhaust gas of the stoichiometric air/fuel ratio flows out from the upstream-side catalyst 43. Then, when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio continues to flow into the upstream-side catalyst 43 and therefore all the oxygen stored in upstream-side catalyst 43 is used, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio begins to flow out from the upstream-side catalyst 43. Then, when it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, the air/fuel ratio of the mixture is brought to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. As a result, exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21. As a result, exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43. It is to be noted herein that since the upstream-side catalyst 43 has the oxygen storage/release capability, oxygen in the exhaust gas flowing into the upstream-side catalyst 43 is stored into the upstream-side catalyst 43, so that exhaust gas of the stoichiometric air/fuel ratio flows out from the upstream-side catalyst 43. Then, when exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio continues to flow into the upstream-side catalyst 43 and therefore the amount of oxygen stored in the upstream-side catalyst 43 reaches a maximum amount of oxygen that the upstream-side, catalyst 43 can store, the upstream-side catalyst 43 can store no more oxygen from exhaust gas, so that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio flows out from the upstream-side catalyst 43. Therefore, it is possible to know the amount of oxygen stored in the upstream-side catalyst 43, that is, the maximum amount of oxygen that the upstream-side catalyst 43 is able to store (hereinafter, referred to as “maximum storable oxygen amount”), on the basis of the amount of time that elapses from when exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio begins to flow into the upstream-side catalyst 43 following the commencement of the control of the air/fuel ratio of the mixture to an air/fuel ratio leaner than the stoichiometric air/fuel ratio to when it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, and on the basis of the degree of leanness of the air/fuel ratio of the mixture obtained when the air/fuel ratio is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, if the maximum storable amount of oxygen is greater than a predetermined threshold value, it can be said that the oxygen storage/release capability of the upstream-side catalyst 43 has not degraded and therefore the upstream-side catalyst 43 is normal. On the other hand, if the maximum storable amount of oxygen is less than or equal to the predetermined threshold value, it can be said that the oxygen storage/release capability of the upstream-side catalyst 43 has degraded.

Therefore, in the internal combustion engine 10, when the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 needs to be determined, the air/fuel ratio of the mixture is firstly controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio. Then, after it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, after it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, the amount of oxygen stored in the upstream-side catalyst 43, that is, the maximum amount of oxygen than the upstream-side catalyst 43 is able to store, that is, the maximum storable amount of oxygen, is calculated on the basis of the amount of time that elapses from when exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio begins to flow into the upstream-side catalyst 43 following the commencement of the control of the air/fuel ratio of the mixture to an air/fuel ratio leaner than the stoichiometric air/fuel ratio to when it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, and on the basis of the degree of leanness of the air/fuel ratio of the mixture obtained when the air/fuel ratio is controlled to the air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, it is determined whether the calculated maximum storable amount of oxygen is greater than the predetermined threshold value. The predetermined threshold value is set at such a maximum storable amount of oxygen that it can be said that the oxygen storage/release capability of the upstream-side catalyst 43 has not degraded. Then, if it is determined that the calculated maximum storable amount of oxygen is greater than the predetermined threshold value, it is determined that the oxygen storage/release capability of the upstream-side catalyst 43 has not degraded. On the other hand, if it is determined that the calculated maximum storable amount of oxygen is less than or equal to the predetermined threshold value, it is determined that the oxygen storage/release capability of the upstream-side catalyst 43 has degraded.

Then, in this embodiment (hereinafter, referred to as “second embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen/release capability of the upstream-side catalyst 43.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the second embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the second embodiment utilizes the flowchart shown in FIG. 14 and a flowchart shown in FIG. 16. The routine shown in FIG. 14 and the routine shown in FIG. 16 are each executed at every predetermined time interval. Incidentally, since the routine shown in FIG. 14 has already been described above, the description of the routine shown in FIG. 14 will be omitted below.

When the routine shown in FIG. 16 starts, firstly in step 300 it is determined whether the flag F2 that shows whether the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied has been set (F2=1). It is to be noted herein that the flag F2 is the same as the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 used in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 300 it is determined that F2=1, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the routine proceeds to step 301. On the other hand, if in step 300 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and the routine proceeds to step 303.

In step 301, to which the routine proceeds after in step 300 it is determined that F2=1, it is determined whether a flag F4 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 (hereinafter, referred to as “upstream-side catalyst degradation determination-purposed rich air/fuel ratio control flag F4”) has been set (F4=1). It is to be noted herein that when the upstream-side catalyst degradation determination-purposed rich air/fuel ratio control flag F4 has been set (F4=1), the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen/release capability of the upstream-side catalyst 43. When the upstream-side catalyst degradation determination-purposed rich air/fuel ratio control flag F4 has been reset (F4=0), it means that the air/fuel ratio of the mixture is not being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen/release capability of the upstream-side catalyst 43.

If in step 301 it is determined that F4=1, it means that the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43, and then the routine proceeds to step 302. In step 302, “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 301 is when in step 300 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1). Therefore, when in step 301 it is determined that F4=1, the condition as the prerequisite for determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, so that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 302, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 301 it is determined that F4=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of degradation of the oxygen/release capability of the upstream-side catalyst 43, and then the routine proceeds to step 303. In step 303, “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 301, it has been determined in step 300 that F2=1, and therefore the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 301 it is determined that F4=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 303, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 300 it is determined that F2=0, the routine proceeds to step 303, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. That is, when in step 300 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 303, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, in the internal combustion engine 10, sometimes the air/fuel ratio of the mixture is set to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55. The determination of the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 as follows.

That is, if the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21. As a result, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55. On the other hand, if the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21. As a result, exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55. Therefore, if the air/fuel ratio of the mixture is changed from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 also changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. In this case, the upstream-side air/fuel ratio sensor output value (i.e., the output value of the upstream-side air/fuel ratio sensor 55) increases from a negative value that corresponds to the air/fuel ratio richer than the stoichiometric air/fuel ratio toward a positive value that corresponds to the air/fuel ratio leaner than the stoichiometric air/fuel ratio. If at this time the upstream-side air/fuel ratio sensor 55 is normal, the upstream-side air/fuel ratio sensor output value increases at a relatively large rate of increase. Therefore, if the rate of increase in the upstream-side air/fuel ratio sensor output value is larger than a predetermined threshold value when the air/fuel ratio of exhaust gas coming to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, it can be said that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is not present and the upstream-side air/fuel ratio sensor 55 is normal. On the other hand, if the rate of increase in the upstream-side air/fuel ratio sensor output value is less than or equal to the predetermined threshold value when the air/fuel ratio of exhaust gas coming to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, it can be said that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is present.

Therefore, in the internal combustion engine 10, when the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 needs to be determined, the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for a predetermined period of time before the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, when the air/fuel ratio of the mixture has changed from the air/fuel ratio richer than the stoichiometric air/fuel ratio to the air/fuel ratio richer than the stoichiometric air/fuel ratio and therefore the air/fuel ratio of exhaust gas coming to the upstream-side air/fuel ratio sensor 55 has changed from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio, it is determined whether the rate of increase in the upstream-side air/fuel ratio sensor output value is greater than the predetermined threshold value. Incidentally, the predetermined threshold value is set at such a rate of increase in the upstream-side air/fuel ratio sensor output value that it can be said that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is not present. Then, when it is determined that the rate of increase in the upstream-side air/fuel ratio sensor output value is greater than the predetermined rate of increase, it is determined that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is not present and therefore the upstream-side air/fuel ratio sensor 55 is normal. On the other hand, when it is determined that the rate of increase in the upstream-side air/fuel ratio sensor output value is less than the predetermined threshold value, it is determined that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is present.

Then, in this embodiment (hereinafter, referred to as “third embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 as described above.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Incidentally, in the internal combustion engine 10, the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 may be performed in the following manner. That is, when the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 is to be determined, an air/fuel ratio control process of controlling the air/fuel ratio of the mixture to an air/fuel ratio richer than the stoichiometric air/fuel ratio for a predetermined period of time and then controlling the air/fuel ratio of the mixture to an air/fuel ratio leaner than the stoichiometric air/fuel ratio is executed a plurality of times, and then, as a final control step, the air/fuel ratio of the mixture is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio. While this control operation is performed, the rate of increase in the upstream-side air/fuel ratio sensor output value is found every time the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes from an air/fuel ratio richer than the stoichiometric air/fuel ratio to an air/fuel ratio leaner than the stoichiometric air/fuel ratio or the other way around. If an average value of the rates of increase thus found (or the smallest rate of the thus-found rates of increase) is greater than the predetermined threshold value, it is determined that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is not present and therefore that the upstream-side air/fuel ratio sensor 55 is normal. On the other hand, if the average of the thus-found rates of increase (or the smallest rate of the rates of increase) is less than or equal to the predetermined threshold value, it is determined that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is present.

In this case, only in the case where it is determined that abnormality in the response of the upstream-side air/fuel ratio sensor 55 is not present and therefore that the upstream-side air/fuel ratio sensor 55 is normal, the presence or absence of the state of inter-cylinder air/fuel ratio imbalance may be determined during the final control step in which the air/fuel ratio of the mixture is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio.

According to this determination process, since the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined on the basis of output values of the normal upstream-side air/fuel ratio sensor 55, the accuracy of the determination can be said to be high.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the third embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the third embodiment utilizes the flowchart shown in FIG. 14 and a flowchart shown in FIG. 17. The routines shown by the flowcharts in FIG. 14 and FIG. 17 are each executed at every predetermined time interval. Since the routine shown in FIG. 14 has already been described above, description of the routine in FIG. 14 will be omitted below.

When the routine shown in FIG. 17 starts, firstly in step 400 it is determined whether the flag F2 that shows whether the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied has been set (F2=1). It is to be noted herein that the flag F2 is the same as the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 used in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 400 it is determined that F2=1, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the routine proceeds to step 401. On the other hand, if in step 400 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and the routine proceeds to step 403.

In step 401, to which the routine proceeds after in step 400 it is determined that F2=1, it is determined whether a flag F5 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55 (hereinafter, referred to as “second abnormality determination-purpose rich air/fuel ratio control flag F5”) has been set (F5=1). It is to be noted herein that if the second abnormality determination-purpose rich air/fuel ratio control flag F5 has been set (F5=1), it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55, and that if the second abnormality determination-purpose rich air/fuel ratio control flag F5 has been reset (F5=0), it means that the air/fuel ratio of the mixture is not being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55.

If in step 401 it is determined that F5=1, it means that the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55, and then the routine proceeds to step 402. In step 402, “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, because when the routine proceeds to step 401 is when in step 400 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), the determination of F5=1 in step 401 means that the condition as the prerequisite for determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 402, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 401 it is determined that F5=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55, and then the routine proceeds to step 403. In step 403, “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 401, it has been determined in step 400 that F2=1, and therefore the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 401 it is determined that F5=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 403, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 400 it is determined that F2=0, the routine proceeds to step 403, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. That is, when in step 400 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 403, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, in the internal combustion engine 10, sometimes the air/fuel ratio of the mixture is set to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started (i.e., for the purpose of so-called warmup the engine 10).

That is, immediately after the internal combustion engine 10 is started, the temperature of the internal combustion engine 10 is relative low, so that fuel supplied into each combustion chamber 21 does not readily burn. Therefore, when the internal combustion engine 10 is started, the air/fuel ratio of the mixture is set to an air/fuel ratio richer than the stoichiometric air/fuel ratio over a certain period of time. Due to this control, since the amount of fuel supplied into each combustion chamber 21 is increased and therefore the amount of heat generated by the combustion of fuel in each combustion chamber 21 increases, the temperature of the internal combustion engine 10 rises relatively quickly, so that fuel favorably burns in each combustion chamber 21 even if the air/fuel ratio of the mixture is controlled to the stoichiometric air/fuel ratio, or controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio.

In this embodiment (hereinafter, referred to as “fourth embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the fourth embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the fourth embodiment utilizes the flowchart shown in FIG. 14 and a flowchart shown in FIG. 18. The routines shown by the flowcharts in FIG. 14 and FIG. 18 are executed when the internal combustion engine 10 is started. Since the routine shown in FIG. 14 has already been described above, description of the routine in FIG. 14 will be omitted below.

When the routine shown in FIG. 18 starts, firstly in step 500 it is determined whether the flag F2 that shows whether the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied has been set (F2=1). It is to be noted herein that the flag F2 is the same as the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 used in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 500 it is determined that F2=1, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the routine proceeds to step 501. On the other hand, if in step 500 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and the routine proceeds to step 503.

In step 501, to which the routine proceeds after in step 500 it is determined that F2=1, it is determined whether a flag F6 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started (hereinafter, referred to as “engine warmup-purpose rich air/fuel ratio control flag F6”) has been set (F6=1). It is to be noted herein that if the engine warmup-purpose rich air/fuel ratio control flag F6 has been set (F6=1), it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10, and that if the engine warmup-purpose rich air/fuel ratio control flag F6 has been reset (F6=0), it means that the air/fuel ratio of the mixture is not being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10.

If in step 501 it is determined that F6=1, it means that the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10, and then the routine proceeds to step 502. In step 502, “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, because when the routine proceeds to step 501 is when in step 500 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), the determination of F6=1 in step 501 means that the condition as the prerequisite for determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 502, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 501 it is determined that F6=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of quickly raising the temperature of the internal combustion engine 10, and then the routine proceeds to step 503. In step 503, “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 501, it has been determined in step 500 that F2=1, and therefore the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 501 it is determined that F6=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 503, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 500 it is determined that F2=0, the routine proceeds to step 503, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. That is, when in step 500 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 503, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, in the internal combustion engine 10, when the output demanded of the internal combustion engine 10 is very small and, in particular, zero, a control of stopping the injection of fuel from the fuel injection valves 25 into the combustion chambers 21 (i.e., a so-called fuel-cut control) is executed. Then, in the internal combustion engine 10, the fuel-cut control is stopped when the output demanded of the internal combustion engine 10 becomes relatively large.

When the fuel-cut control is being executed, the air/fuel ratio of the mixture is an air/fuel ratio that is much leaner than the stoichiometric air/fuel ratio. Therefore, exhaust gas whose air/fuel ratio is much leaner than the stoichiometric air/fuel ratio is discharged from each combustion chamber 21, so that the exhaust gas whose air/fuel ratio is much leaner than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43. That is, during execution of the fuel-cut control, since exhaust gas whose air/fuel ratio is much leaner than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43, it is highly likely that the upstream-side catalyst 43 will store a large amount of oxygen from the exhaust gas flowing into the catalyst 43 and the amount of oxygen stored in the upstream-side catalyst 43 will reach a maximum storable oxygen amount (i.e., a maximum amount of oxygen that the upstream-side catalyst 43 is able to store). In this case, if exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43 after the fuel-cut control is stopped, the upstream-side catalyst 43 cannot store oxygen from the exhaust gas that flows into the catalyst 43. That is, the upstream-side catalyst 43 cannot exert its oxygen storage/release capability. On the other hand, if exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43, the upstream-side catalyst 43 releases oxygen that has been stored therein, as described above. Therefore, when the amount of oxygen stored in the upstream-side catalyst 43 has reached the maximum storable oxygen amount, it is appropriate to send exhaust gas of a rich air/fuel ratio into the upstream-side catalyst 43 so that the upstream-side catalyst 43 releases oxygen that has been stored therein. After that, if exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43, the upstream-side catalyst 43 can store oxygen from the exhaust gas.

Therefore, in the internal combustion engine 10, immediately after the fuel-cut control is stopped, the air/fuel ratio of the mixture is set to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release oxygen that has been stored in the upstream-side catalyst 43.

That is, in the embodiment (hereinafter, referred to as “fifth embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release stored oxygen after the fuel-cut control is stopped as described above.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the fifth embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the fifth embodiment utilizes the flowchart shown in FIG. 14 and a flowchart shown in FIG. 19. The routines shown by the flowcharts in FIG. 14 and FIG. 19 are executed when the fuel-cut control is stopped. Since the routine shown in FIG. 14 has already been described above, description of the routine in FIG. 14 will be omitted below.

When the routine shown in FIG. 19 starts, firstly in step 600 it is determined whether the flag F2 that shows whether the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied has been set (F2=1). It is to be noted herein that the flag F2 is the same as the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 used in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 600 it is determined that F2=1, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the routine proceeds to step 601. On the other hand, if in step 600 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and the routine proceeds to step 603.

In step 601, to which the routine proceeds after in step 600 it is determined that F2=1, it is determined whether a flag F7 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release stored oxygen after the fuel-cut control is stopped (hereinafter, referred to as “stored oxygen release-purpose rich air/fuel ratio control flag F7”) has been set (F7=1). It is to be noted herein that if the stored oxygen release-purpose rich air/fuel ratio control flag F7 has been set (F7=1), it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release oxygen stored therein, and that if stored oxygen release-purpose rich air/fuel ratio control flag F7 has been reset (F7=0), it means that the air/fuel ratio of the mixture is not being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release oxygen stored therein.

If in step 601 it is determined that F7=1, it means that the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release oxygen stored therein, and then the routine proceeds to step 602. In step 602, “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, because when the routine proceeds to step 601 is when in step 600 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), the determination of F7=1 in step 601 means that the condition as the prerequisite for determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 602, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 601 it is determined that F7=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of causing the upstream-side catalyst 43 to release oxygen stored therein, and then the routine proceeds to step 603. In step 603, “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 601, it has been determined in step 600 that F2=1, and therefore the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 601 it is determined that F7=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 603, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 600 it is determined that F2=0, the routine proceeds to step 603, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. That is, when in step 600 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 603, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, when the output demanded of the internal combustion engine 10 is very large, the temperature of exhaust gas discharged from each combustion chamber 21 becomes very high. If such a very high temperature exhaust gas continues to flow into the upstream-side catalyst 43, there is possibility of the temperature of the upstream-side catalyst 43 becoming very high and therefore the upstream-side catalyst 43 undergoing thermal degradation. On the other hand, if an exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio flows into the upstream-side catalyst 43, vaporization of fuel in the exhaust gas on the upstream-side catalyst 43 removes heat from the upstream-side catalyst 43 and therefore the temperature of the upstream-side catalyst 43 declines.

Therefore, in the internal combustion engine 10, when the output demanded of the internal combustion engine 10 is very large, sometimes the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43.

In this embodiment (hereinafter, referred to as “sixth embodiment”), the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large.

According to this determination process, since the air/fuel ratio of the mixture is not controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, the fuel economy of the internal combustion engine 10 improves in comparison with the case where the air/fuel ratio is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

Next, an example of a routine of executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in accordance with the sixth embodiment will be described. The determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the sixth embodiment utilizes the flowchart shown in FIG. 14 and a flowchart shown in FIG. 20. The routines shown by the flowcharts in FIG. 14 and FIG. 20 are executed when the fuel-cut control is stopped. Since the routine shown in FIG. 14 has already been described above, description of the routine in FIG. 14 will be omitted below.

When the routine shown in FIG. 20 starts, firstly in step 700 it is determined whether the flag F2 that shows whether the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is satisfied has been set (F2=1). It is to be noted herein that the flag F2 is the same as the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 used in step 200 in FIG. 15. Therefore, if the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. If the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been reset (F2=0), it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied.

If in step 700 it is determined that F2=1, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied, and the routine proceeds to step 701. On the other hand, if in step 700 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied, and the routine proceeds to step 703.

In step 701, to which the routine proceeds after in step 700 it is determined that F2=1, it is determined whether a flag F8 that shows whether the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large (hereinafter, referred to as “catalyst temperature-lowering purpose rich air/fuel ratio control flag F8”) has been set (F8=1). It is to be noted herein that if the catalyst temperature-lowering purpose rich air/fuel ratio control flag F8 has been set (F8=1), it means that the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43. Besides, if catalyst temperature-lowering purpose rich air/fuel ratio control flag F8 has been reset (F8=0), it means that the air/fuel ratio of the mixture is not being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43.

If in step 701 it is determined that F8=1, it means that the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of lowering the temperature of the upstream-side catalyst 43, and then the routine proceeds to step 702. In step 702, “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, because when the routine proceeds to step 701 is when in step 700 it is determined that the inter-cylinder air/fuel ratio imbalance determination prerequisite condition flag F2 has been set (F2=1), the determination of F8=1 in step 701 means that the condition as the prerequisite for determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied and the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance can be accurately determined. Then, the routine proceeds to step 702, in which “1” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=1, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is performed.

On the other hand, if in step 701 it is determined that F8=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of purpose of lowering the temperature of the upstream-side catalyst 43, and then the routine proceeds to step 703. In step 703, “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. After that, the routine ends. That is, when the routine proceeds to step 701, it has been determined in step 700 that F2=1, and therefore the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has been satisfied. However, when in step 701 it is determined that F8=0, it means that the air/fuel ratio of the mixture is not being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and therefore that the present state is a state in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance cannot be accurately determined. Then, the routine proceeds to step 703, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

Incidentally, if in step 700 it is determined that F2=0, the routine proceeds to step 703, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. Then, the routine ends. That is, when in step 700 it is determined that F2=0, it means that the condition as the prerequisite for executing the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance has not been satisfied. Then, the routine proceeds to step 703, in which “0” is input to the inter-cylinder air/fuel ratio imbalance determination execution flag F1. In this case, in step 100 in the routine shown in FIG. 14, it is determined that F1=0, so that the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is not performed.

By the way, when the mixture burns in the combustion chambers 21, hydrogen (H₂) is produced. The amount of hydrogen produced by the combustion of the mixture is larger when the mixture whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is burned than when the mixture whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio is burned. Therefore, the amount of hydrogen in the exhaust gas discharged from the combustion chambers 21 is larger when the mixture whose air/fuel ratio is richer than the stoichiometric air/fuel ratio is burned than when the mixture whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio is burned. That is, the exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio contains a larger amount of hydrogen than the exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio.

The molecules of hydrogen are smaller than the molecules of oxygen (O₂), carbon monoxide (CO), and hydrocarbons (HCs). Therefore, the hydrogen in the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 diffuses in the diffusion resistance layer 559 at higher speed than the oxygen, carbon monoxide and hydrocarbons contained in the exhaust gas. Then, the hydrogen affects the amount of oxygen ions that flow in the solid electrolyte layer 551 of the upstream-side air/fuel ratio sensor 55. Specifically, in the comparison between the case where an exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55 and the case where an exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55, the amount of oxygen ions that flow in the solid electrolyte layer 551 of the upstream-side air/fuel ratio sensor 55 is larger when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is richer than the stoichiometric air/fuel ratio than when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 is leaner than the stoichiometric air/fuel ratio provided that the degree of richness of the rich air/fuel ratio of the exhaust gas relative to the stoichiometric air/fuel ratio is equal to the degree of leanness of the lean air/fuel ratio of the exhaust gas relative to the stoichiometric air/fuel ratio. In other words, the characteristic of output of the upstream-side air/fuel ratio sensor 55 varies greatly according to the amount of hydrogen in the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55, that is, according to the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55.

When the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio and exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio alternately comes to the upstream-side air/fuel ratio sensor 55. Therefore, in that case, if the characteristic of the output of the upstream-side air/fuel ratio sensor 55 varies greatly according to the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55, the air/fuel ratio of the mixture cannot be accurately controlled to the stoichiometric air/fuel ratio even if the air/fuel ratio of the mixture is intended to be controlled to the stoichiometric air/fuel ratio on the basis of the output value of the upstream-side air/fuel ratio sensor 55.

Therefore, in order to prevent the output characteristic of the upstream-side air/fuel ratio sensor 55 from varying according to the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55, it is appropriate to employ an air/fuel ratio detection element 55 a as shown in FIG. 21 which is similar to the air/fuel ratio detection element 55 a shown in FIG. 12 but has a catalyst 561 in the through holes 558. The catalyst 561 is disposed so as to close the through holes 558. Besides, the catalyst 561 is a porous body, and supports a catalytic substance that accelerates oxidation-reduction reactions and an oxygen storing material that achieves an oxygen storage/release capability, as in the upstream-side catalyst 43.

In the air/fuel ratio detection element 55 a shown in FIG. 21, the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 flows into the diffusion resistance layer 554 through the catalyst 561. When the exhaust gas passes through the catalyst 561, the catalyst 561 accelerates the oxidation of hydrogen in the exhaust gas, thereby lessening the amount of oxygen in the exhaust gas. According to this arrangement, when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio comes to the upstream-side air/fuel ratio sensor 55, the amount of hydrogen in the exhaust gas is lessened before the exhaust gas flows into the diffusion resistance layer 554. As a result, the variation of the characteristic of the output of the upstream-side air/fuel ratio sensor 55 commensurate with the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 can be eliminated.

Incidentally, the functions of the air/fuel ratio detection element 55 a illustrated in FIG. 21 are the same as the functions of the air/fuel ratio detection element 55 a shown in FIG. 12, except that exhaust gas flows into the diffusion resistance layer 554 through the catalyst 561.

In the case where the upstream-side air/fuel ratio sensor 55 includes the air/fuel ratio detection element 55 a that is equipped with the catalyst 561 shown in FIG. 21, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio achieves more effects, compared with the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance performed while the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio.

That is, in the case where the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio, exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio and exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio alternately comes to the upstream-side air/fuel ratio sensor 55, as described above. In this case, in order for the upstream-side air/fuel ratio sensor 55 to output values that very accurately correspond to the air/fuel ratio of exhaust gas that comes to the sensor 55, the flow direction of oxygen ions flowing in the solid electrolyte layer 551 of the air/fuel ratio detection element 55 a of the upstream-side air/fuel ratio sensor 55 needs to instantly reverse when the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 changes between the rich side of the stoichiometric air/fuel ratio and the lean side of the stoichiometric air/fuel ratio. However, the direction of flow of oxygen ions in the solid electrolyte layer 551 does not instantly reverse, as described above. In the case where the air/fuel ratio detection element 55 a of the upstream-side air/fuel ratio sensor 55 has the catalyst 561 in the through holes 558, the direction of flow of oxygen ion in the solid electrolyte layer 551 reverses even less readily. Therefore, when the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio, the output value of the upstream-side air/fuel ratio sensor 55 is even less likely to very accurately show the air/fuel ratio of the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 (i.e., the air/fuel ratio of the mixture).

However, if the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, the exhaust gas that comes to the upstream-side air/fuel ratio sensor 55 at that time is only the exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio, and therefore it is not required that the direction of flow of oxygen ions in the solid electrolyte layer 554 of the upstream-side air/fuel ratio sensor 55 reverse during the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance. Therefore, in the case where the upstream-side air/fuel ratio sensor 55 includes the air/fuel ratio detection element 55 a that is equipped with the catalyst 561, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance performed while the air/fuel ratio of the mixture is being controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio achieves more effects, compared with the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance performed while the air/fuel ratio of the mixture is being controlled to the stoichiometric air/fuel ratio.

Incidentally, in the foregoing embodiment, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance utilizes the absolute value of an average slope of a line that is followed by the upstream-side air/fuel ratio sensor output value (i.e., the output value of the upstream-side air/fuel ratio sensor 55), that is, the absolute value of the time-dependent rate of change in the upstream-side air/fuel ratio sensor output value.

However, instead of using the absolute value of the time-dependent rate of change in the upstream-side air/fuel ratio sensor output value, the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance may use, for example, the largest value of the absolute values of values of the first order time differential of the upstream-side air/fuel ratio sensor output value, the largest value of the absolute values of values of the second order time differential of the upstream-side air/fuel ratio sensor output value, or the length of the signal trace between two different upstream-side air/fuel ratio sensor output values with a predetermined time interval therebetween.

In the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the largest value of the absolute values of values of the first order time differential of the upstream-side air/fuel ratio sensor output value (hereinafter, referred to as “maximum value of the first order time differential of the upstream-side air/fuel ratio sensor output value”), the maximum value of the first order time differential of the upstream-side air/fuel ratio sensor output value is larger when the state of inter-cylinder air/fuel ratio imbalance is present than when the state of inter-cylinder air/fuel ratio imbalance is not present. Therefore, in the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the maximum value of the first order time differential of the upstream-side air/fuel ratio sensor output value, the largest value of the maximum values of the first order time differential of the upstream-side air/fuel ratio sensor output value that are obtained when it has to be determined that the state of inter-cylinder air/fuel ratio imbalance is not present is determined beforehand as a threshold value. If the maximum value of the first order time differential of the upstream-side air/fuel ratio sensor output value is greater than this predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is present. If the maximum value of the first order time differential of the upstream-side air/fuel ratio sensor output value is less than or equal to the predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is not present.

In the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the largest value of the absolute values of values of the second order time differential of the upstream-side air/fuel ratio sensor output value (hereinafter, referred to as “maximum value of the second order time differential of the upstream-side air/fuel ratio sensor output value”), the maximum value of the second order time differential of the upstream-side air/fuel ratio sensor output value is larger when the state of inter-cylinder air/fuel ratio imbalance is present than when the state of inter-cylinder air/fuel ratio imbalance is not present. Therefore, in the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the maximum value of the second order time differential of the upstream-side air/fuel ratio sensor output value, the largest value of the maximum values of the second order time differential of the upstream-side air/fuel ratio sensor output value that are obtained when it has to be determined that the state of inter-cylinder air/fuel ratio imbalance is not present is determined beforehand as a threshold value. If the maximum value of the second order time differential of the upstream-side air/fuel ratio sensor output value is greater than this predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is present. If the maximum value of the second order time differential of the upstream-side air/fuel ratio sensor output value is less than or equal to the predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is not present.

Besides, in the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the length of the signal trace between two different output values of the upstream-side air/fuel ratio sensor (hereinafter, referred to as “length of the signal trace of the output of the upstream-side air/fuel ratio sensor”), the length of the signal trace of the output of the upstream-side air/fuel ratio sensor is longer when the state of inter-cylinder air/fuel ratio imbalance is present than when the state of inter-cylinder air/fuel ratio imbalance is not present. Therefore, in the case where the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance uses the length of the signal trace of the output of the upstream-side air/fuel ratio sensor, the largest value of the lengths of the signal trace of the output of the upstream-side air/fuel ratio sensor that are obtained when it has to be determined that the state of inter-cylinder air/fuel ratio imbalance is not present is determined beforehand as a threshold value. If the length of the signal trace of the output of the upstream-side air/fuel ratio sensor is longer than this predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is present. If the length of the signal trace of the output of the upstream-side air/fuel ratio sensor is less than or equal to the predetermined threshold value, it is determined that the state of inter-cylinder air/fuel ratio imbalance is not present.

Incidentally, the foregoing manners of the determination of the presence or absence of the state of inter-cylinder air/fuel ratio imbalance in the first to sixth embodiments may be combined as appropriate. Therefore, the embodiment of the invention may be a construction in which in an internal combustion engine in which the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for at least one of the purposes of: determining the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56; determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43; determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55; quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started; causing the upstream-side catalyst 43 to release stored oxygen therefrom after the fuel-cut control is stopped; and lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large, the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined while the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio.

Besides, in the internal combustion engine 10, if there is a case where the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for a purpose that is other than the purposes of: determining the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56; determining the presence or absence of degradation of the oxygen/release capability of the upstream-side catalyst 43; determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55; quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started; causing the upstream-side catalyst 43 to release stored oxygen therefrom after the fuel-cut control is stopped; and lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large, and that is also other than the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, it is also permissible to determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance while the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio. Therefore, the embodiment of the invention may also be a construction in which in an internal combustion engine in which the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for a purpose other than the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance, and in which the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined while the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for a purpose other than the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

In particular, in the internal combustion engine 10, in order to determine the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56, the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio over a predetermined period of time. On the other hand, in the internal combustion engine 10, in order to determine the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43, firstly the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio, and then the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio when it has been detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43. Specifically, both when the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56 is to be determined and when the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 is to be determined, the air/fuel ratio of the mixture is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio. Therefore, if the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56 and the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 are determined in a series of processes, good efficiency is achieved.

Therefore, in the internal combustion engine 10, the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56 and the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 may be determined as follows. That is, in the internal combustion engine 10, when the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56 and the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43 are to be determined, firstly the air/fuel ratio of the mixture is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio over a predetermined period. The predetermined period herein is set at a time that is sufficiently long for the oxygen stored in the upstream-side catalyst 43 to run out when exhaust gas whose air/fuel ratio is richer than the stoichiometric air/fuel ratio continues to flow into the upstream-side catalyst 43. Then, when the foregoing predetermined period elapses after the air/fuel ratio of the mixture begins to be controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is determined whether the output value of the downstream-side air/fuel ratio sensor 56 shows an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio.

If it is determined that the then downstream-side air/fuel ratio sensor output value (i.e., the then output value of the downstream-side air/fuel ratio sensor 56) shows an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is determined that abnormality in the output of the downstream-side air/fuel ratio sensor 56 is not present and that the downstream-side air/fuel ratio sensor 56 is normal. On the other hand, if it is determined that the then downstream-side air/fuel ratio sensor output value does not show an air/fuel ratio of exhaust gas that corresponds to the air/fuel ratio of the mixture that is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, it is determined that abnormality in the output of the downstream-side air/fuel ratio sensor 56 is present. When the predetermined period elapses after the air/fuel ratio of the mixture begins to be controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio, the air/fuel ratio of the mixture is controlled to an air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, when it is determined by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, the amount of oxygen stored in the upstream-side catalyst 43, that is, the maximum amount of oxygen that can be stored into the upstream-side catalyst 43, that is, the maximum storable amount of oxygen, is calculated on the basis of the amount of time that elapses from when exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio begins to flow into the upstream-side catalyst 43 following the beginning of the control of the air/fuel ratio of the mixture to the air/fuel ratio leaner than the stoichiometric air/fuel ratio till when it is detected by the downstream-side air/fuel ratio sensor 56 that exhaust gas whose air/fuel ratio is leaner than the stoichiometric air/fuel ratio has begun to flow out from the upstream-side catalyst 43, and on the basis of the degree of leanness of the air/fuel ratio of the mixture obtained when the air/fuel ratio is being controlled to the air/fuel ratio leaner than the stoichiometric air/fuel ratio. Then, it is determined whether the calculated maximum storable amount of oxygen is greater that a predetermined threshold value. The predetermined threshold value herein is set to a maximum storable amount of oxygen of such a degree that it can be said that the upstream-side catalyst 43 of the oxygen storage/release capability has not degraded. Then, if it is determined that the calculated maximum storable amount of oxygen is greater than the predetermined threshold value, it is determined that degradation of the oxygen storage/release capability of the upstream-side catalyst 43 is not present. On the other hand, if it is determined that the calculated maximum storable amount oxygen is less than or equal to the predetermined threshold value, it is determined that degradation of the oxygen storage/release capability of the upstream-side catalyst 43 is present.

By the way, there is known a so-called hybrid system as shown in FIG. 22 which includes an electric motor M in addition to the foregoing internal combustion engine 10 in order to produce drive force for driving a vehicle. The hybrid system shown in FIG. 22 has a drive force switching mechanism P for switching the transmission path for the drive force for driving a vehicle 70 according to the state of travel of the vehicle 70, and a transmission TM that transmits the drive force transmitted thereto from the drive force switching mechanism P to a drive force transmission system of front wheels 71 of the vehicle 70.

The electric motor M is an alternating-current electric motor, and is driven by alternating-current electric power supplied from an inverter I that converts direct-current electric power supplied from a battery B into a predetermined alternating-current electric power. Besides, drive force switching mechanism P is able to switch the transmission path for drive force among a mode in which the transmission path for drive force is established only between the electric motor M and the transmission TM, a mode in which the transmission path for drive force is established only between the internal combustion engine 10 and the transmission TM, and a mode in which the transmission path for drive force is established between the electric motor M and the transmission TM and between the internal combustion engine 10 and the transmission TM.

In this hybrid system, in the case where in the internal combustion engine 10, the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose of determining the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56, or the purpose of determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43, or the purpose of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55, or the purpose of quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started, or the purpose of causing the upstream-side catalyst 43 to release stored oxygen therefrom after the fuel-cut control is stopped, or the purpose of lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large, it is possible to achieve more effects than in the case where the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined when the air/fuel ratio of the mixture is controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio only for the purpose of determining the presence or absence of the state of inter-cylinder air/fuel ratio imbalance.

That is, as described above, in the hybrid system, the transmission path for drive force is sometimes established only between the electric motor M and the transmission TM. In such cases, operation of the internal combustion engine 10 is stopped. Therefore, the occasions on which the internal combustion engine 10 is operated are correspondingly less frequent in the hybrid system, so that it can be said that in the hybrid system, there are less frequent occasions on which the air/fuel ratio of the mixture in the internal combustion engine 10 can be controlled to an air/fuel ratio richer than the stoichiometric air/fuel ratio. If in the internal combustion engine 10, the presence or absence of the state of inter-cylinder air/fuel ratio imbalance is determined simultaneously with the control of the air/fuel ratio of the mixture to an air/fuel ratio richer than the stoichiometric air/fuel ratio which is performed for the purpose of determining the presence or absence of abnormality in the output of the downstream-side air/fuel ratio sensor 56, or of determining the presence or absence of degradation of the oxygen storage/release capability of the upstream-side catalyst 43, or of determining the presence or absence of abnormality in the response of the upstream-side air/fuel ratio sensor 55, or of quickly raising the temperature of the internal combustion engine 10 when the internal combustion engine 10 is started, or of causing the upstream-side catalyst 43 to release stored oxygen therefrom after the fuel-cut control is stopped, or of lowering the temperature of the upstream-side catalyst 43 when the output demanded of the internal combustion engine 10 is very large, it is possible to achieve an effect of correspondingly increasing the frequency of the occasions to determine the presence or absence of the state of inter-cylinder air/fuel ratio imbalance. 

1. A multicylinder internal combustion engine comprising: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers; a downstream-side air/fuel ratio sensor disposed in the exhaust passageway downstream of the exhaust control catalyst so as to detect the air/fuel ratio of the exhaust gas that flows out from the exhaust control catalyst; a control device that executes a first abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the downstream-side air/fuel ratio sensor is abnormal; and a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the first abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.
 2. The multicylinder internal combustion engine according to claim 1, wherein: the control device executes a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal; and the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the second abnormality determination-purpose rich air/fuel ratio control is being executed.
 3. The multicylinder internal combustion engine according to claim 2, wherein the determination device executes the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.
 4. The multicylinder internal combustion engine according to claim 1, wherein the control device executes an engine start-time rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when operation of the multicylinder internal combustion engine is started, a post-fuel injection stop rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when injection of fuel from the fuel injection valves is re-started after the injection of the fuel from the fuel injection valves is stopped, or an exhaust control catalyst-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when temperature of the exhaust control catalyst is higher than a predetermined permissible upper-limit temperature; and the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the engine start-time rich air/fuel ratio control is being executed, or when the post-fuel injection stop rich air/fuel ratio control is being executed, or when the exhaust control catalyst-purpose rich air/fuel ratio control is being executed.
 5. A multicylinder internal combustion engine comprising: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers; a control device that executes a second abnormality determination-purpose rich air/fuel ratio control of controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio when it needs to be determined whether the upstream-side air/fuel ratio sensor is abnormal; and a determination device that executes an inter-cylinder air/fuel ratio imbalance determination of estimating the air/fuel ratio of the mixture formed in each combustion chamber based on an output of the upstream-side air/fuel ratio sensor when the second abnormality determination-purpose rich air/fuel ratio control is being executed, and of determining whether there is a difference between the air/fuel ratios of the mixtures that are estimated.
 6. The multicylinder internal combustion engine according to claim 5, wherein the determination device executes the inter-cylinder air/fuel ratio imbalance determination if it is determined that the upstream-side air/fuel ratio sensor is not abnormal when the second abnormality determination-purpose rich air/fuel ratio control is being executed.
 7. The multicylinder internal combustion engine according to claim 6, wherein: the control device executes an engine start-time rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when operation of the multicylinder internal combustion engine is started, a post-fuel injection stop rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when injection of fuel from the fuel injection valves is re-started after the injection of the fuel from the fuel injection valves is stopped, or an exhaust control catalyst-purpose rich air/fuel ratio control of controlling the air/fuel ratio of the mixture formed in each combustion chamber to an air/fuel ratio richer than the stoichiometric air/fuel ratio when temperature of the exhaust control catalyst is higher than a predetermined permissible upper-limit temperature; and the determination device executes the inter-cylinder air/fuel ratio imbalance determination when the engine start-time rich air/fuel ratio control is being executed, or when the post-fuel injection stop rich air/fuel ratio control is being executed, or when the exhaust control catalyst-purpose rich air/fuel ratio control is being executed.
 8. A multicylinder internal combustion engine comprising: a plurality of combustion chambers; fuel injection valves disposed corresponding to the individual combustion chambers; an exhaust control catalyst disposed in an exhaust passageway so as to remove a specific component of exhaust gas discharged from the combustion chambers; an upstream-side air/fuel ratio sensor disposed in the exhaust passageway upstream of the exhaust control catalyst so as to detect air/fuel ratio of the exhaust gas discharged from the combustion chambers; a control device that controls the air/fuel ratio of a mixture formed in each of the combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference between in the air/fuel ratio of the mixture between the combustion chambers.
 9. An inter-cylinder air/fuel ratio imbalance determination apparatus comprising: an upstream-side air/fuel ratio sensor disposed in an exhaust passageway upstream of an exhaust control catalyst so as to detect air/fuel ratio of exhaust gas discharged from a plurality of combustion chambers of a multicylinder internal combustion engine, the exhaust control catalyst being disposed in the exhaust passageway so as to remove a specific component of the exhaust gas discharged from the combustion chambers; a control device that controls the air/fuel ratio of a mixture in each of the plurality of combustion chambers to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers; and a determination device that determines whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on an output of the upstream-side air/fuel ratio sensor when the air/fuel ratio of the mixture is being controlled to the air/fuel ratio richer than the stoichiometric air/fuel ratio for the purpose other than the purpose of determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers.
 10. An inter-cylinder air/fuel ratio imbalance determination method comprising: determining whether a first condition for determining whether there is a difference in air/fuel ratio between a plurality of combustion chambers of a multicylinder internal combustion engine is satisfied; determining whether a second condition for controlling the air/fuel ratio of a mixture formed in each combustion chamber to an air/fuel ratio richer than a stoichiometric air/fuel ratio for a purpose other than a purpose of determining whether there is a difference in the air/fuel ratio between the combustion chambers; detecting the air/fuel ratios of exhaust gas discharged from the plurality of combustion chambers of the multicylinder internal combustion engine; and controlling the air/fuel ratio of the mixture in each of the combustion chambers to the air/fuel ratio richer than the stoichiometric air/fuel ratio when the first condition and the second condition are satisfied, and then determining whether there is a difference in the air/fuel ratio of the mixture between the combustion chambers based on the detected air/fuel ratios. 